Larry, throughout my eight or so years in the trade I've only worked in only a small handful of GE panels and don't completely understand their buss set up. I'm almost too embarrassed to ask but the twin looking breaker that has the handle tie, is that a slim version of a double pole?
Yes. Start with the panel bus stabs. Most tandems have a single body like a 1p breaker, and have the same kind of contact as a 1p, with the bus blade horizontal (as you look at the mounted panel.)
GE bus tabs have a little 'cross-arm' on each stab, and their tandems each contact one side of the cross-arm, which is why their 'tandems' are really separate, individual 1/2"-thick, half-space breakers.
If you look at the underside of a GE breaker, the bus stab slot is really an X, and the contacts themselves have little slots, so the little cross-arms don't prevent the breaker from seating fully.
In a typical tandem breaker, both halves are supplied by the same bus stab, because they fit where a 1p breaker fits. The GE half-space breakers contact only their own cross-arm, and not the main blade.
The GE 2-pole half-space breakers are simply two half-space bodies riveted together, just like standard 2-pole breakers (excluding CH, QO, etc.), and the bus stab contacts are separate.
So, GE THQP 2p breakers are able to obtain 240v because they are installed straddling the phases, with each half contacting one phase bus, just like quad breakers, which are tandem 2p breakers.
Either another 2p half-sized breaker (which would contact the next stab, also), a single half-space breaker, or a 1/2" filler, takes up the other half of the 1" space. This is similar to Federal Pacific panels.
Added: I used an indoor version of that panel for my home-theater sub-panel:
