I am not fully sure how to backfil so I wont try figuring that one out unless someone has a solid guide on it, but sealing at vacuum seams doable would seal it at like 10 or so? Looking at the instructions I am not sure if I need to put the red back fill adaptor on both tubes or just one, I have two of the red adaptors just in case.
If its both do you need to try and unscrew both at the same time so that when its unscrewed from one it doesn’t have much time to suck in air three the tee’d connection.
The Backfill Adapter is designed to provide a temporary sealed pathway into an enclosure through a Pressure Relief Valve (PRV) - it isn’t designed to seal off a tube going into it when it’s disconnected from the PRV, so I don’t expect using it with a tee’d connection would work particularly well.
It would likely make more sense to backfill enclosures (or pull them to vacuum) one at a time, rather than trying to do multiple at once. In the “pull to vacuum” case there may be some pressure transfer through incoming cables (e.g. from connected enclosures, devices, and the tether), although that should hopefully not be excessive, especially at weak vacuum pressures.