if i do two muscle groups in a day i will start with one like biceps and do all my sets for bi's and then do tri's. The next time I do arms I will start with tri's then do bi's. The muscle that starts the workout first for me sees the best results, so its good to alternate. For my delts i always did fronts, sides and rears. I did that for 4 months and the fronts were huge, sides were ok and rears had no growth. So I switched it up to start with rears first and in one month my rears grew like a weed! Finally looking better!