If one pool has 51% of the hash rate, it will still have 51% of the hash rate even if all the other pools are merged together.
A pool can only mount a "majority attack" if it can control the transactions that are included in the blocks that its miners work on. Is that necessarily the case? If each miner in the pool was free to get the block's transactions (except the generation one) from any other node, then the malicious pool owners would need to obtain the complicity of enough miners to make 51% of the hash rate. Is it possible to force pools to leave that freedom to their miners?
On the other hand, if the two largest pools each have 26% of the hash rate, they could still conspire to mount a 52% attack.
