Using the kryotherm calculator and aiming to deal with a 600w heatload and assuming magic cooling that keeps any hot side at 30c. Aim is always total COP of 1:

-16x 226 modules gives dT ~26c at 9v

or

-16x 172 modules gives dT ~26c at 8v

Stacking becomes tricker to work out:

Top layer:

-16x 226 modules dealing with 900w heat input

-COP3 at 5.3v gives dT ~10c so cold side ~20c

Bottom layer:

-16x 172 modules dealign with 600w heat input

-COP2 at 6.3v gives dT ~15c with 20c hotside

So stacking example gives total dT of 25c at COP1, slightly worse than a single layer of either type of TEC (not to mention other losses made worse by stacking).

-16x 226 modules gives dT ~26c at 9v

or

-16x 172 modules gives dT ~26c at 8v

Stacking becomes tricker to work out:

Top layer:

-16x 226 modules dealing with 900w heat input

-COP3 at 5.3v gives dT ~10c so cold side ~20c

Bottom layer:

-16x 172 modules dealign with 600w heat input

-COP2 at 6.3v gives dT ~15c with 20c hotside

So stacking example gives total dT of 25c at COP1, slightly worse than a single layer of either type of TEC (not to mention other losses made worse by stacking).