where did you get the 2nd 4g sim card?
one came with the iPhone 6.
The other one was from the other phone on the account. I don't think that matters because I wasn't able to swap them. System wouldn't let me activate that way.
When all was said and done, all I did was transfer the upgrade off my unlimited account to another account and activate it on the unlimited line. For whatever reason I wasn't forced to switch plans.
I went through a bunch of other steps but I don't know that it wouldn't have worked exactly the same if I didn't go through those steps.
Edit- actually I think first activating the iphone 6 on the non-unlimited account was somewhat beneficial. Because when I was later activating it on the unlimited line I was "activating a device previously on the account". That could be why I wasn't asked to pick a new plan on the unlimited line.