the two tachs don't need to be going simultaneously, you just need to plug your aftermarket tach in to the same tach input lead as the stock gauge. This will verify that the input signal is good, and its not the igniter.
Is your speedo cable driven? If so, it may be the cable is a bit sticky that makes the speedo jump. Mine does that, I wouldn't worry about it.
As far as the oil pressure goes, thats a good question as I have yet to get my gauge working properly. For the longest time I just used the old switch to drive the gauge. If there was no oil pressure the gauge would read high, if there was oil pressure the gauge would show zero. But again, that was using an oil light sender with a gauge. If you aren't sure the gauge works properly, test it. The oil sender is really just a variable resistor to ground. So, pull the wire off the sender, creating an open, the gauge should read low. Now, ground the wire to the sender, the gauge should read high. If so, its working. If not, check your oil level. Or maybe check your oil level first . . .
The best way to make sure, is to install a mechanical oil pressure gauge.
