For twinset use, personally I'd go for the Apeks DS4 1st stages with ATX50 second stages (mainly 'cause I like the shiny bits on them.
) Don't bother spending any more money on the ATX100 or 200 varients as you're not gonna get any real benefit!
Never been a big fan of the TX regs as I found they used to give me jaw ache. Not that impressed by the newer XTX 2nds either as I think they're over engineered - give me a nice lightweight, simple ATX reg any day. I've got about five sets which get used on stage & deco bottles, my twinset and my singles rig. Can't beat 'em!
That said, I'd probably choose a Scubapro MK25 1st stage over the DS4 for a singles rig - the hose routing is much nicer! Someone on here did mention that Apeks are bringing out some new 'Tek3' 1st stages designed specifically for twinset divers but I'll be darned if I can find out any more about them! It could all be bollards...