In this scenario, I think, the potential danger is that the hash power ie 60% estimated between 2 pools is greater than the rest of the hash power and the forked chain cannot keep up and cannot become the longer chain. Longest chain is one of the consensus rules I believe. Although in writing this I can see the solution is just to get people to point their miners at diff pools lowering their hash power and of course diversifying the number of pools.
Discussion
The longest chain with integrity wins, miners that detect a longer chain with a double spend do not have to give up a join the longer chain. They can very happily keep mining on the shorter chain they are on. Effectively become blind to any chain with integrity issues. Part of giving up and switching to a longer chain is a simple integrity check on that chain.
All distributed systems consensus base have to handle split brain.
Implication: It may take more than 6 confirmations to be considered irreversable. But not much more…