Location: Many. From NE salt, Russia, Canadian A Salmon, NW Steelhead, Bahamas, Keys. Live in Upstate NY
Probably the most important similarity is smoothness. Another is back cast (D loop) 180° from target. Differences: one of the most important factors in spey is placing the grip — and that there are almost countless ways of accomplishing it (depending on current, side of river, wind direction, etc.)
I would agree with Bill that smoothness is so important with both single and double handed rods. When I finally learned to slow down while spey casting, that helped my single handed casting immensely. On the differences, I would say that the idea of constantly increasing tension on the line throughout the entire cast is important (at least for traditional type casts...non-skagit). As opposed to the load..unload...load...unload of false casting a single hander.
In casting the principals for a two-hander are the same as in single hand, application of power, straight rod tip path, stop, pause, anchor placement (single/two-hand) etc.
The only difference would be, understanding the roll the lower hand plays (power application) and, hauling (single hand power application)
Grip, stance would need to be discussed as possible differeances also.
Spey Q #2 -- What to tell beginning Spey casters that have some single-hand experience about the similarities and differences between Spey and single-hand casting?
Fundamentals of a fly cast: loading move, power-snap [stop], drift.
Five major differences:
A “D” loop
Back cast 180 degrees from target
In all back casts the line is thrown under the rod.
In all back casts the rod movement ends in an upward direction
[The first three credit Simon G; the last two credit Way Yin.]
Be prepared for casters with some single-hand experience to not understand the fundamentals of a fly cast.