I have done as you described (SPG attached to LPI). It is really nice in that you can just glance down at the SPG. It does not dangle as it is a short DIR length hose. It is a bit awkward as the extra hose adds resistance to the LPI hose if you are using it to vent gas. Also, when I grab the LPI sometimes the hose made it slower to grip it and vent with gloves on since it is a good sized bundle of hoses, a significant drawback although most of the time I vent with the butt dump.
I still have a preference to clip it to the left chest d-ring, routing the hose under my left arm. Still can read without un-clipping and seems to be the best solution for me since the LPI is then free and clear. Since I don't sling stages there is nothing conflicting.
I tried the left waist d-ring, frankly I wasted too much time clipping and unclipping with thick gloves. I tried using an old retractor, it lasted a few dives and died how all my others did, prematurely.