Veldy, if two pools can collude, than so can three, four and more. This risk is inherent to bitcoin design. The only solution to this problem is monitoring and notifying miners in case when pools start to double spend.
It appears to me that by the time double spending is detected you have two problems. Somebody has already lost their money and it is already too late. I am confident that an attack would be executed in seconds with the initial spend and the blocks in the pool (s) wouldn't be dispensed until the new chain was longer than the original ... so by the time it is detected it is too late.