I have both.  A LL and a RR.  

What caliber would you put where?

I'd love to shoot 556, 9mm and 7.62x39 (in that order).  

I'm thinking of setting up the RR for rifle calibers and the LL for 9mm.  

I've read some where that the 9mm may put more stress on the RR so that's why I ask.  

Once they are set up and rocking, I'll doubt I'm going to mess with them.  If I do mess with it, I'll mess with the LL lower.

Yeah probably a good idea to use the RLL for anything that would possibly cause damage to the lower. Not sure if that's even possible though, do they make 9mm BCG that have the proper SP1 cut?

Not made but can be machined to work.  I have a couple of them now.  Working on fitment and timing now.

I use LL on Ares MCR & 5.56 cheap lowers.  
Steel DIAS on 9mm and anything else.

Why so?   What's your thought process?