I have seen it done on a Super 73. But only on a Youtube video and without any word of whether it survived. Someone very recently did it to a Sondors Madmods and we'll see how that goes.
Speaking personally as someone who has built a lot of bikes, and a bunch of 2wd hub+mid drive bicycles, and who rides them as daily drivers, its a bad idea.
Just for starters, you are going to put probably a 120 to 160 Nm motor in charge of yanking your chain. That chain leads back to either a cassette body that is secured to a plate on the hub motor with what are probably 6 (or 8) tiny M3 screws. Tearing that motor cover clean off, or at least warping it, is a possibility. Have you seen what a mid drive can do to a cassette body on a proper hub? Wears it out faster unless you get a steel upgrade. If its steel, the pawls underneath are the next failure point, and you can bet a hub motor built with the expectation that the drivetrain is both light duty (hub motors dramatically reduce stress on the drivetrain) and human-powered only is not beefed up like it could have been. With that thought in mind, do you know where to get a replacement cassette body for your hub motor? Chances are you'd have to buy another motor and swap in a new motor core.
What if you have a freewheel? Well the tearing-off and warping risk is still there for the rear motor cover, and you are adding to that the fact you need to put your faith in the threads that screw the freewheel onto the motor. I've seen them strip on Bafang motors under strong human effort. If they don't give way, at the very least they are going to screw that bottom-of-the-line chinesium freewheel onto the motor mighty tight, making the inevitable (accelerated) replacement of same a bigger adventure than usual (google "destructive freewheel removal Park Tool" to see the tutorial).
Taking a machine-built wheel with short spokes not meant for that extra load may not work out so great in the long run, either.
So... your perception of whether this idea will work will not fully be formed until you've tried riding it for awhile and see what ends up breaking. Initially I'm sure it will seem fun.
If you want to do two motors on a new project, do two wheel power. If you are trying to give your existing hub motor some guts going up hiils, I'd either start over with a new back wheel and a mid drive, or sell the bike and start over.
OK. You've talked me out of another bad idea. I'm officially giving up the idea of taking an inexpensive ebike and adding a motor or substituting a bigger motor to get some more acceleration and top speed.
I will start looking for a bike that fits me and already has a 1500 or 2000 watt rear hub motor in it.
My dreams are shattered!
You can still do that. Putting two motors on one wheel is the bad idea.
2016. Sondors Original ebike, 350w, 15a controller, 36v, $700 delivered:
2017. Same bike. Sort of. Upgrades everywhere. 2wd on twin 750w motors, twin 35a controllers, two 52v batteries (an 8ah brick is in the bag on the handlebars), new stronger wheels, hydraulic brakes.
2018: A pothole impact cracked the frame at the rear lower chainstay where the rack attaches to it... the frame was the one thing I could never upgrade unless I had bought a better bike in the first place.
So, I have been there, done that and made this mistake myself. If you are going to start shoveling money, think it through first (or do what you did and ask the peanut gallery so you can hear some advice given in hindsight).