Drop the current of a couple of peristaltic pumps for a microscope use
As a first step, since you want to control the speed of the motors, you want to control voltage, not current. I would suggest replacing the resistors with a variable voltage power supply, standard lab equipment. (You might leave a low value variable resistor in series with each motor, normally turned to minimum R, so you can slow down one motor if it's slightly faster than the other to compensate for mechanical differences). Then the power supply can vary the speed of both motors together.

However, that won't work over a huge range. I'd guess it'll work down to 10-20% of rated speed, i.e. 4-8mL/min whereas you need to get down to about 1% of rated speed. It may be better than this, depending on the design of the motors themselves (friction etc) but controlling speed reliably down to 1% is optimistic.

I think you may need to consider interposing a 4:1 gearbox between motor and pump. If you're lucky, something suitable will be available from stock from a motor supplier such as Maxon, otherwise some mechanical engineering is required.

Alternatively, for greater precision at more complexity, replacing the motors with stepper motors and suitable drive circuitry would allow you to control the pump speed down to individual steps (1/200 revolution) with any precision you need.