Does anyone know the policy of combining multiple Thankyou accounts?

I have 3 thankyou accounts from 3 cards (Forward, Premier and AT&T), they are under the same website login, but they show 3 different thankyou points account.

I know sharing points will trigger the 60 days expiration policy, but what if I combine the 3 accounts into one? I do see there is "Combine my thankyou member accounts". Will this also trigger 60 days expiration policy, or just keep each card's separate expiration date?

Thanks!