Assume it's going to be the way Stephanie Meyer writes.
I have never read nor watched anything of hers, and I had to google to even know for sure who she is, but I'm still insulted.
So going through the Time Portal twice probably isn't an option, unless you're tricky about it somehow.
I wonder how the Enveloping Pit (and its contents) would interact with the portal.
The space inside the pit is another dimension. The pit itself is a mobile doorway. So it'd work pretty much as you suspect. That's what I was thinking about when I said "tricky". It didn't take you long to come to the same idea, heh.