In general, it sounds like those RoboClaw motor controllers should capable of running a load like the heaters you mention. The maximum driver output voltage might be a few tens of mV less than the input voltage at the current you mention, but that generally does not seem like it would require a compensation in the supply. Also, it should be possible to control several of them on the same USB bus, but generally you would use something like a USB hub so they each have their own USB port. There is a packet serial mode on the RoboClaw that can allow more than one controller on a single serial port, but you might contact Basicmicro to see if that requires any additional hardware like a USB to TTL converter and how connections would need to be made for that.
There are many ways you might go about controlling heaters like that. Our High-Power Simple Motor Controller G2 18v15 should offer you a similar level of control to the RoboClaw and we have instructions for daisy-chaining multiple devices through a single serial port in the Serial daisy chaining section of the controller’s User’s guide. The cost for 6 channels (which would require 6 simple motor controllers) is slightly more expensive than the RoboClaws you mentioned, but pretty close.
If you are comfortable writing your own firmware for a microcontroller like our Arduino compatible A-Star programmable controllers or a switching frequency longer than a couple of seconds is sufficient to control your heaters, we might be able to offer other suggestions.