Yeah not very practical. It just gets complex. (Just like this reply did for me...)
In theory if there were a DZ with tons of funding (eg military) you could be lent a reserve canopy set up to use as a main. "You have a Smart 150 in your rig? Well, we have a PD 143 set up here, that would at least be similar." There would be complexities because a reserve doesn't normally have the deployment bag attached. (So you need a specially built reserve, or static lining the jump, or removable deployment system, or someone else chasing the d-bag.)
Sometimes there are big boogies where manufacturers bring reserves set up to be tried out as mains, but then you need to be around such a big skydiving event.
But if you want to actually cut away from a parachute, then you need a 3rd canopy, a reserve, on the system. You can have the reserve to test in its proper place on your back -- which makes putting a real reserve on your belly more complex, especially to have it fully legal. Or you could have the reserve to test on you belly, which keeps your 'last reserve' in your rig as normal, but then the deployment for the test canopy won't be like a real reserve.
Either way, having 3 canopies makes the gear and handles and procedures and crap that your wearing more complex and less suited to a newbie. Maybe more dangerous than a real cutaway after a mal!
(There was even a World Champion doing a stunt jump for a commercial about 25 years ago, with 3 canopies, who screwed up the order he pulled stuff and died. An unusual case but 3 canopies does get complex.)
And even if you set up everything to cutaway to a reserve to test flying it, there won't be nearly the same stress level as if you were having an actual malfunction. So then ideally you'd at least do something like pop one toggle on the main to get yourself spinning around before cutting away.
Reserves do fly a little different than the ZP canopies people are used to today. A small F-111 style canopy will tend to have a shorter, sharper flare motion, not a long gradual flare motion. It used to be that people were used to F-111 style canopies from their student days, but now they don't get that. So I do get a little concerned about newer jumpers these days knowing how to properly flare their reserve. At least people learn that they should do practice flares under their reserve when actually flying it after a malfunction.
All in all, it gets complex. So in the sport it is considered reasonable to just spend one's time practicing on the ground. Hanging harnesses are good, handle checks on all jumps are good. And you don't buy a reserve that is way smaller than what you are used to jumping as a main.