The Democrats will put two separate bills on the floor: the bipartisan infrastructure bill and the human infrastructure bill.
If Mitch and the gang want to vote no on the former, then they can. Then the Democrats get all the credit for both.
Biden’s calculation is that the GOP has to agree on the former and agree to disagree on the latter. Doing so allows Biden to claim some measure of bipartisanship, and allows Mitch to share credit for infrastructure (immensely popular with voters), yet distinguish the GOP on human infrastructure as too fiscally irresponsible.