Couple of installation things.
A pedal assisted BBS02, unless you reprogram its settings, will run much faster in all pedal assist levels than a small 36V hubmotor, so the hubmotor will probably not keep up and its effort will just be wasted power. So some tuning work needed to make it rideable in pedal assist.
Also some install work. The BBS02 has an internal PAS sensor. If you are currently running pedal assist on the hubmotor, you'll have to mount its sensor on the BBS02 bracket if you want to keep it, That will probably need some home made brackets.
If you plan to use a throttle on both motors, unless you've got some electrical design expertise, a shared throttle is some work. Most people opt for dual throttles. You can get used to it.
You don't necessarily need a 36V and a 48V battery, if the 36V kit can run on 48V, but you will need a big one to deliver 30-35A. Probably cheaper to run two batteries.
When you're all done, I don't know what you really gain. AT least if you had a front drive hub motor, there could be some traction gains. If looking for top speed, I think the gains are minimal as the BBS02 can spin the wheel faster than a small geared hubmotor.
