Chromis are schooling fish and when they are grouped together in our smaller tanks they definitely have a pecking order. You can start with 8-10 of them and most of the time they will slowly get picked ojn to the point that you lose them over time. Being that you only have 2 in the tank, one is probably messing with the other, and it just felt safer hanging with the clowns.
I have a rule of thumb about chromis that seems to work well and its 1 chromis / 30g tank volume. For better results also feed multiply times a day and feed cyclopeeze or rotifers or some small planktonic foods like that one of the times daily or at least every other day.
I believe the chromis killing eachother off has to do with space and food.
I have 7 chromis who school and never lost one in over a year since i bought my first 5 i just recently added 2 more and all is well a month later and these were big chromis and still no fighting.