The existing ultras are two max dies connected together with TSMC’s CoWoS-S interposer. But as I understand the interposer can have yield issues, so yes — you put two together, but it’s not quite as easy as snapping together legos.

It used to mean that, but the new M5 Pro and M5 Max have separate CPU and GPU chiplets with an interposer, similar to how the previous generation Ultras were based on connecting two Max full dies. So it's unclear whether there will be any Ultra for the M5.