Splitting a relatively long paper into two shorter papers I have recently submitted a 40 pages paper to a journal, say (A). After about 6 months, the editor let me know that several reviewers have declined to review my paper, and so he decided to reject the paper. He suggested that I submit my paper to a more specialized journal. Journal (A) is already a specialized journal and I only know 1 journal more specialized than (A), let's call it journal (B). So, one of my options is to submit my paper to journal (B) and accept the risk of a similar feedback from the editors of journal (B), of course after several months. 

In the mean while, I think the main reason several reviewers declined to review my paper is that (1) my paper is relatively long, (2) My paper consists of two parts and each part addresses a different subject. Therefore, the set of reviewers who have expertise in both subjects and are willing to read and review my paper is very small. Due to these facts, it is very likely the editors of journal (B) face the same problem. So, as the second option, I am thinking of splitting my paper into two shorter papers each consisting only one subject of my original paper. Regarding this option, I can think of the following pros and cons:

Pros:

(i) There are a good number of experts in each subject and it is fairly easy to find a reviewer for each one of my shorter papers.

(ii) This facilitates the referee process of each paper and hopefully reduces its time period .

(iii) Two papers (each approximately 20 pages) look better than one paper (approximately 40 pages) in my CV. 

Cons:

(a) The second part of my paper depends on the notations and results of the first part. So the reviewer of the second part may prefer to read and review the whole paper at once, or even worse he/she may call the paper containing the second part incomplete.

(b) Part of the motivation of the developments in the first part of my paper comes from my work in the second part. By separating these two parts, the reviewer of the first part can complain about the lack of enough motivations and justifications for my results. In my opinion, it is not a serious problem because I will explain the application of my works which is going to appear in the second paper. But I am not the person who makes the final decision and the reviewer may blame this and reject the paper.

(c) I can imagine that it would be a difficult path to follow the referee processes of two related papers simultaneously, because of the following reasons: It is possible that the opinions of reviewers of the shorter papers differ significantly. Or it is possible that these papers get refereed in two very different time periods. It is also possible that one of the papers gets accepted and the other one doesn't, which is a pretty ugly situation.

Unfortunately, I have faced each of the above difficulties ((a), (b) and (c)) in my previous submissions and I know how they can ruin my papers. In fact, the main reason that I organized my results collectively in one paper was to avoid the above issues. But now that my paper has been rejected without any peer review, I am considering the option of splitting my results into two papers. So, I have the following questions to ask from people who have more experience and have been involved with similar situations (for instance as an editor or a referee):

(1) What do you think about the above pros and cons? Do you know any other pros and/or cons? And, is it a good idea to split my paper into two shorter papers?

(2) If it is advisable to split my paper into two papers, should I submit them to the same journal, (maybe same editor), or should I submit them to different journals according to the best editors who can handle my papers?
Well, in my opinion, the following fragment of your question outweighs everything else:


  (2) My paper consists of two parts and each part addresses a different subject. Therefore, the set of reviewers who have expertise in both subjects and are willing to read and review my paper is very small.


I appreciate that you are able to imagine the problem that will be faced by the journal editors. So, splitting into two is certainly not a bad idea in this case.

I can mention some instances where this has been done in the past. In each of these cases, the problem wasn't as acute as yours (i.e. both could actually be reviewed by the same expert), but perhaps the authors chose to do this because of length considerations (or some other reason that I can't imagine). Also, in these cases, it wasn't a case of two different journals A and B - it was two sequential papers in the same journal. So, that creates an additional option for you, if you find it appropriate. (The context here is Physics, but I'm sure this can be generalized to Maths, if I'm right!)

Example 1: R. P. Feynman was a charismatic Nobel Laureate (Nobel, 1965), as you probably know. Here are his two significant contributions to QED, appearing back to back in Physical Review:

Paper1, Paper2 (both are free pdfs officially, given their landmark status.)

In particular, he began Paper 1 by writing:


  This is the first of a set of papers dealing with the solution of problems in quantum electrodynamics.


and started Paper 2 with the sentence:


  This paper should be considered as a direct continuation of the preceding one ...


He had developed the formalism in the former and applied it to the problem in the latter. That makes a candidate for splitting into two. 

Example 2 Here are two papers by Sidney Coleman which form the backbone of phenomenological effective Lagrangian method in low-energy Nuclear Physics. (These aren't free and I'm not sure you will be able to get past the paywall here!) 

Paper 1, paper 2. 

Notice again, that they are consecutive papers in the same journal. The second paper also has an extra author, and that could be one reason for splitting into two. But once again, from the point of view of content, the general method was devised in paper 1 and applied to some context in paper 2. But here, the authors spent a section of paper 2 in explaining what they developed in Paper 1.

Thus, long story cut short - it should be possible to go ahead and split into two parts. If they are consecutive, you can carry over everything directly, if not, spend a few sentences explaining your notation etc. 

PS - Congratulations for doing this sort of work which could put the editors into this type of a fix. That smells like a significant contribution to Maths, having applications elsewhere (other branches?), which is probably why you insist that it would be rare to find a referee who can ably judge both!