How to reply to an unofficial offer from my second choice when I also have an unofficial offer from my first choice? Here is my situation. I am looking for a post-doc position. I have two choices, A and B. 

My first choice is A. And I already got an un-official offer from A. I was told that A is working on a official offer. But I haven't got the official offer yet. 

Now, my second choice, B, also send me an email. B tells me that he would like to give me an offer if I am interested in joining his lab. How should I reply? Is it appropriate to tell him that I am interested?
If you're interested in both, then email each of them and say so. 

If you don't have the formal offer then you don't have the job and shouldn't make decisions without it.