Some sensors have contacts that can handle the high currents of fan(s), usually these are the mechanical type with the small diameter capillary tube from the sensor to the control unit, often with a temp adjustment feature. I don't like these, or even the electronic sensing types that require a tube or wire to sneak past the radiator hose clamping area and into the water flow - like where the hose goes from thermostat housing to radiator. Too much risk of corrosion and leaks.
Relays. Any controller that doesn't specify current capability (Amps) that is at least the total of your two fans, should not be used without a relay. The relay is just a magnetic controlled switch. The magnet actuator in it (a coil) draws a relatively small current from the sensor unit and translates this to high current capable contacts that switch +12V from the battery (thru a fuse for protection) to the fans. Any number of fans can be controlled by one relay – just hook each fan to the other; red to red, black to black (called a parallel circuit), as long as the total demand doesn't exceed the contact rating. Any number of separate relays can be used, one for each fan, as long as the sensor unit output current rating is big enough for the relay energizing coils (total of all relay inputs).
Solid state relays can also be used. They have essentially a light source (light emitting diode) adjacent to a light activated component that handles the higher current demand of the source (fan in this case). There are no moving parts or contacts to corrode. Just be careful of the polarity so you don't let the smoke out (blow it up).
So, I would look for a kit that had a sensor that either screws into the cooling system like your temp gauge, has an adjustable set point so the fans come on after the thermostat opens but before the engine gets too hot (180 or so) and has the current capacity for your two fans combined. If you find a setup that doesn't have the current capability, use the setup as a switch to energize your relay, which actually switches the fans. Many auto stores and electrical supply houses have the relay for the job. Something like a 12v input (coil or solid state type) and output capability of 25 Amps (or greater than both of your fans).
Try googling "Hayden". They make fans and thermostat kits. They have a setup that senses the radiator externally. I have also seen setups on EBay from England that clamp a small sensor onto the radiator hose externally. Again, I would prefer a sensor that taps directly into the water passage in the block or intake manifold area near the thermostat.
Also, let's ask Marshall what the original sensor setup is in the 2500M. We could adapt from there. If there's a place in the plumbing to get at the water, I'll help you find a sensor that will screw in, with the appropriate control unit.
Send me more details of your fans and I can assist further - even buy the relay for you if necessary..
Sorry for the long winded explanation. This is why my kids don't ask me how something works....