Atomic Swap - What if redeem is done just before expiration
As you stated, setting 2 different expiration times provides safety.

My recommendation is to select the network with the longest period before immutability as the first contract, and pick a time that is agreeable with both parties. 

The second contract on the other chain should use an expiry that is half of the first contract. This gives the redeemer on the first blockchain some time before the expiry.