Must say that sidemount with two cylinders isn't nice in my experience.
If going to normoxic depths (~60m/200'), those deco cylinders need to be ali80s so that you've enough reserves to cover one failure. Sidemount with 4 full-sized cylinders (2 sidemount, 2 deco) is horrid (IMHO). Steve Martin makes it look oh so easy, but they're utter bs'tards to handle. Steve also uses sliding bottom clips to tighten them into the body. If you don't do that, they dangle horribly.
If you're only using a second deco stage as a backup for shallower depths, circa 40m/130', then you can use ali7s or even ali40s if they're similar gasses. These are easier to handle but again dangle unless you tighten everything up.
Personally, backmount is far nicer for normoxic depths.