If you're using an existing gage, you'll need sensors that answer to that gage, and a "DP3T" or "SP3T" switch (depending on whether you want to switch grounds as well or not - most sensors use the chassis proper as a ground reference, so the 3PST should serve.)
If you're using an add-on gage, you'll want a spare sensor (for the second signa) and either a "DPDT" or "SPDT" switch.
SWITCH JARGON -
Most switches are labelled as "xPyT" - read as "X Poles Y Throws."
A "pole" is a circuit element to switch - for instance, if you need to switch two wires, you'll need a "dual pole" switch to switch both wires. If you're just switching a signal WRT (="with respect to,") ground, or just powering one of two circuits, then a "single pole" switch will work. "SP" means single (one) pole, "DP" means dual (two) poles, more than that just uses the number (4P is a four-pole switch, or four terminals being switched Y times.)
A "throw" is an option on the switch - "ST" is an "on-off" switch, usually. "DT" means dual throw - the input (poles) can be switched to one of two outputs. "DPDT" is a switch that will flip two leads to one of two "on" positions. As with poles, above - if there are more than two, the number is just used ("3T" has "three throws," or three "ON" positions.)
Digging a little further...
The typical XJ temperature sensor (RENIX and OBD-I, the one in the cylinder head) is a self-grounding unit - meaning it uses the mounting threads to get its ground reference WRT the chassis, like when it's screwed into the cylinder head. So, you'll have to make sure that the fitting the sensor is screwed into has either a good ground (easiest to do,) or that you pull the ground to the switch (requiring an extra wire to go the full run, and the use of a double pole switch, vice a single pole.) Barring the ground, the sensor can be mounted in any convenient location.
Then, you pull the two additional signal wires to the same location as the OEM temperature signal, and pull all three of them to a SP3T switch (assuming you grounded the sensors locally, vice grounding them through the switch. A "local" ground is, you probably thought, a ground near the sensor.) One "throw" will be the OEM sensor, another will be the tranny cooler inlet, and the third will be the tranny cooler outlet, say.
If you use an add-on gage (which I suggest - IP gages are more "relative" than "absolute" units...) then you just need to order a spare sensor for the gage, and note whether it is self-grounding or grounds through the circuit/switch.
If it's self-grounding, any SPDT toggle will do - those are typically "centre off" switches, so just decide which input you want with the switch "up" and the other with the switch "down."
If it grounds through the switch and gage, you'll need to use a DPDT toggle - the signal goes to one "pole" and the ground to the other. Just like wiring an SPDT, but you've got an extra wire to deal with.
Single- and Double-throw switches can be knobs or toggles, but more poles typically mandates the use of a rotary (knob) switch. It's also possible to "cascade" switches - have a DPDT, say, to switch between the OEM input and the ones you added, then have another to switch between added inputs - but that's more work than you really want to get into - although it is entirely doable.
If you want to break it down a little more to make it easier to understand, think this way...
The typical light switch in your house is SPST - it switches power to the lamp or not. One pole, one throw.
If you've got two switches that can operate a single light (hallways are good for this,) they're probably both SPDT - you only need to switch power, but the switches can go either way and have two "on" settings. They're wired so you just have to switch one to match the other one to turn the lamp ON, but switching either one from the ON position will also turn the lamp OFF. I could go into it farther, but I'd need pictures to explain it easily.
A relay (like you've probably got underhood,) is typically used as an SPST switch - you just use a small switch (low load, long wiring, transistor, and such) to turn the relay ON and OFF, and then the relay handles the large current. This is what takes the load off of your ignition switch when you start your vehicle - the key handles about a quarter amp, but the relay is good for 15 or so with the start motor solenoid.
Make any more sense now?
