Yes, because these are separate transactions, and as far as we know there is no logic in the systems to do more.
a) Activate subsidized new phone on tiered data line. System checks, this is ok, because it's not unlimited and new phone is compatible with plan. So this goes through.
b) Activate phone on unlimited line. This phone is (apparently) not a subsidized upgrade, as that happened on the other line, so this is a simple phone activation. Again, phone is compatible with plan (e.g. not a basic phone on a smartphone plan or vice versa), so that also works.
So while this could in theory be changed (e.g. no activation on unlimited on phone previously activated on tiered within the last X days) it would take work and money, and probably not enough people do this for Verizon to worry. But, of course, I am not guaranteeing this will work when you try it!