36" is really long for the left tank in sidemount, most people I know use around 24". I wouldn't think of the two regs as primary and secondary, you're switching back and forth so think of them as either right/left or long hose/short hose. I like slightly longer SPG hoses; I use 9" instead of the more common 6". I find it easier to read them. 9" for the inflator hoses is fairly common, 6" is also common at least with the razor.
I put a 90 angle swivel on the short hose and leave the long hose straight. It's very comfortable, and this way it's almost instantaneous to identify which reg you are breathing from, you just reach up and feel if there's a swivel. This makes it very quick and idiot-proof for air sharing and/or valve drills.
If you learn SM from a good instructor, for example someone who dives SM in caves expertly and is also a cave instructor, you should become very proficient at all the standard skills; reg swap/clip, S drills, reclipping AL tanks (you'll need 2 sets of D rings on your hip belt or sliding D rings, which I hate) deploying and stowing the long hose, dropping the right tank, etc. (There are many more) There's a lot to learn just for efficient gear up if you're paying attention to all the details. My SM class, which was for full cave diving, took a solid 4 days and that was after I had more than 50 full cave dives (plus many intro level dives and training dives) in BM doubles.
You can certainly use SM doubles in OW, but the task loading to be really good at it is much more in the technical diving realm of skill level. The whole point of SM is to have flexibility in problem solving and restriction penetration. You can 'do more' (I guess) with a SM rig in the water, and this means that there's more tasks to learn how to do, all with excellent trim, buoyancy, and propulsion control. So I'm not sure you can truly separate 'rec' sidemount from 'tech' sidemount, other than, of course, the other demands of the dive, like overhead or deco demands. I guess my point is that even in nice shallow warm clear OW, good thorough SM training involves a level of skill mastery and task loading more typically associated with technical diving. So you shoudl learn a lot more than just how to rig a SM set up.