Maybe this seems odd, but at 7 months you just put them in a room (or separate rooms for twins) close the door and they will fall asleep eventually. If your wife is actively calming them down then they will not learn to nap, only learn how to get her attention.
This is something my wife and I read about but did not have the inner strength to do until our 3rd child and we found ourselves overwhelmed at nap AND bedtime.
My oldest is 11 now and youngest 6 and they are doing good! (Not napping anymore, but letting them cry it out hasn't turned them into resentful children or anything like my wife and I used to think).
As a final note, my last girl did not have a favorite "blankey" like the first two did. So, I made sure she had the same blanket all the time and eventually it helped sooth her.
At first there were days where they would be in their cribs awake for the whole time. Sometimes crying (like an hour straight). After a couple days they crying got much less and nap time began to be a blessing for all of us.
While I was very apprehensive about 'abandoning them' the professionals were right about them surviving and growing to adapt to this new routine. They also learned to be alone and that Mom and Dad would always be back for them.