You're gonna find that the information posted on mods is information for those that know the answers to those questions you ask or have knowledge of amps. Guys like Dozin and Waldo have tirelessly answered them before for people that don't know much about it, so I would look in the archives of posts deeply, not just looking casually, but using a bunch of searches. I am sure what you are asking is there.
I can tell you that I have a TRRI and the wiring has both channels tapped off of one point since the circuit is a bit different. seeing that I also shunted after the tap too, the power section is spared the load also. I assume that both the taps go to one jack since you will only be preamping one guitar at a time. If nothing is plugged into the normal channel it is not sending a signal to interfere with the vibrato. If you only use the vibrato channel then only tap that point. You can remove an RCA jack and put a 1/4 inch in there or drill a new hole. I think dozin used one of the second input 1/4" of his vibrato channel. You could feasibly use the RCA's too if you desire.
Go ahead and do the mod, you can always use the twin as a combo with both taps no matter what.