My supervisor wants to put someone else as first author on one of two papers when I have done most of the work on both. What should I do? I have completed experimental research on a novel topic. I was the instigator of the direction of the research and the one to find the results. Then, theoretical work was performed to support my experiments. While the theory confirmed all my work, there was nothing original or new in the theoretical support. I wrote the paper and drafted the manuscript. 

The group supervisor wants the theory paper to have the theory researcher as first author. The group leader's arguments is that "there will be two papers, so share the first authorship". 

I think this is unfair. Why should I have to share my work just because there will be two papers out of it?!

I am asking for guidance on this. In particular as scientific supervisors, is it fair to put the main researcher as an "et al", just because someone else in the group has not got a paper?
Generally speaking, it is infeasible for us to judge whether authorship or the order of authors on any given project is "fair", as all of those depend a great deal on discipline and the particularities of any given project.

That being said, from the little text you provide the 1/1 split does sound fair. There are two papers coming out of the work, one experimental and the other theoretical. The person who did the experiments first-authors the experimental one, the "theory guy" leads the theory paper. So far, so good, I would say.

You say that there was "nothing new" in the theoretical work. This seems highly unlikely to me, what would even go into the theory paper if that was indeed the case? In general, providing a solid theoretical basis to experimentally observed results is a very valuable contribution, that you maybe need to learn to value higher. Sometimes the theoretical methods to achieve this are straight-forward, but this does not diminish their value for your project.