If i have two flying figures placed like this

fig1 | rooftop
ground | fig2

where fig1 is on the ground, and fig2 is on the roof, can either figure taxi the other without using a move action to just change elevation?

Thanks

Nope. Even though they are in adjacent squares, they are technically not adjacent because of the elevation difference.

Per the FAQ on Adjacent:

"Squares on different elevations are not adjacent to each other."

Hope this helps!