The TX650 V2 is the superior unit and it would be overkill even if you added a 2nd 6950. All that would really be needed is a quality-made 550W unit:
With two 6950s under full load in their system, their PSU pulled 473W from the wall outlet. Their CPU was idling, so I'll add 150W for an overclocked 2500K. So a simultaneous full load between the 6950s and the 2500K would make the PSU pull 623W from the wall outlet. If the PSU is 85% efficient while pulling 623W from the wall outlet, then that would mean the system is pulling about 530W from the PSU.
Here is a very high-quality modular PSU that would be more than enough for this:
The 550W SeaSonic G Series at Scan.co.uk for £73.86 inc. VAT
It can continuously deliver 550W "24/7" if it's ever needed. ;)
Edited by TwoCables - 12/10/12 at 8:14pm