temp is a voltage gauge so there would be no problem just putting two in parallel. The tach works off of pulses so again, no problem with paralleling them. You seemed to indicate that the oil pressure is a mechanical gauge so you would need to put a "T" in the line from the engine
to the gauge and just run a line from the "T" to the other oil pressure gauge. Judicious use of some anti-chaffing tape/conduit or something to insure that a hole doesn't get rubbed in it would be a good idea.
Ain't nothing left to do but do it. Good luck.