The 84 volts is not the required voltage, it is the voltage that will give the maximum possible speed out of your motors.
Your motors will run fine at 48 volts but will not have as high a maximum speed.
The rest of you machine may not be capable of the maximum speed as well and you may not need the faster speed.
If your motors are 4 wire than you have no choice in how to connect but if they are 8 wire then hooking up in bipolar parallel will give the lowest inductance.