Undermybed

Hi there I hope not to make a repetition, but I've searched and I didn't find any topic with this information. I have 2 heads - H&K Tubemeister 18w and Peavey Valveking 20w - and bought a second-hand M13. I want to use it connected to both fx loop. Which method would be more effective? For a first approach I used the input and output and it works, but (if possible) I would like to use as fx 1 and 2 for one head, and fx 3 and 4 for the other head. Anyway, I accept suggestions.