High torque in a front motor makes the wheel lose traction, especially on hills. It's even worse with 20" wheels because the torque is proportionally higher than with larger wheels.
You need to tell us about what speed you want, what weight you have and what sort of hills you need to go up rather than focusing on a single characteristic.
To answer your question, it's probably one of the Heinzmann motors. They run at about 28 amps. They state that their cargo motor can pull 300 kg up a 13.1% hill. That's with a 250w motor!!!!!
It takes 981w to lift 100kg at 1m/s. lets say the bike would climb at 8 mph. That's 3.6m/s, so on a hill of 13.1 %, the vertical speed would be 0.468m/s. Therefore the power needed to lift that 300kg bike up the hill would be 3 x 981 x .468 = 1377 watts of output. That's quite a powerful 250w motor then, when you consider that a Bosch CX gives around 500w when climbing. Input power would be over 2kw.
Wait, it gets even better. They make twin 125w motors that are certified to EN15194 when combined, and can pull 500kg up a 15.8% hill with their combined 250w. This seems to answer the question of whether multiple motors are allowed. You just need to find the right 125w ones.
The CargoPower system developed specially for cargo bikes features innovative technology, a robust design and excellent reliability.
www.heinzmann-electric-motors.com