Ah, most bigger motors have 4 brushes. These work together as 2 pairs of contacts , each pair doing half of the windings.

This is very obvious in the magmotors, who have 4 wires coming out.
I believe you can drive a Magmotor S28-150 with a pair of TZ85's, @ 5S

In the Bosch it's done by connecting the pairs on the brush-holder. And that is what Marto suggests, decouple the pairs, and control each pair with a TZ85.

For non-overvolted wheelchairs, a TZ85 will do fine, especialy when you add a fan to the speedo's. Don't expect any world shattering speed or acceleration, but it will move, and if done well, you can build yourself a cheap bot that can take serious punishment. Just use thick enough steel and shock mounting. A nice bodyshell with a cool paintjob, and you'll have a great starter heavy.