You can only split the oil pressure, not the water temp. The tip of the sender has to stick out in the water flow. If you use a T, like you would with the oil pressure, the coolant will get trapped in the T, and the temp won't be accurate.

Also, consider using electrical gauge as opposed to the mechanical gauges. I can't seem to get mechanical water temp gauges to work for more than a few months in my truck before they die. With a mechanical oil pressure gauge, you have to worry about the line leaking or rupturing. I have mech. gauges now, and I'm in the process of swapping them out for electrical ones.