the new dual die thing they’re promoting for how they make the M5 Pro and M5 Max chips?

It's chiplets just like GB10, Strix Halo, etc. One die has the CPU and the other die has the GPU.

How is that different from the silicon [bridge] they were using before?

It's probably similar.

the two dies don’t have to fabbed next to each other

They never were; this is a widespread misunderstanding.

But would this affect the interconnect speed somehow?

Apple never documented the internal interconnect for the M4 Pro/Max and now they don't document it for the M5 Pro/Max so we don't know. It's probably better to read reviews and avoid theorycrafting and backseat driving.