Attack vectors for difficulty retargeting with BCH and BTC?
Short answer: You can protect with money.

It is an economic problem. First, if the marketcap changes slowly, the difficulty can adopt. Now suppose the marketcap changes suddenly and miners behave rationally for the short term profit.

Since total_reward = block_reward + fee, a sufficient higher fee can attract miners. For example, if the marketcap on one side is double, then the compensation fee should be more than 12.5 BTC. This total fee may seem high, but it had actually happened. During the time lacking miners, there is 13.4 BTC in block #494045, and many other blocks with 11+ BTC. This is around $30 per transaction and people are willing to pay it! Hence, for the "protection", rich people can pay high fee, poor people can make donation transaction, and "irrational" miners can stay by losing profit. 

Anyway, you cannot use the blockchain if you cannot pay a high fee. Note that the above strategy is true in both directions: A "protection" on one side is an "attack" on the other side. So the fee on both side may rise exponentially together.