If you are holding a totem of undying in both hands, the one in your offhand will be used first. In Java Edition, items in the main hand are preferred over items in the offhand.

Steps to reproduce:

1. Get 2 totems of undying, and put one in your offhand and one in your main hand. Make sure that the currently-selected hotbar slot is the one with the totem.
2. Switch to survival mode.
3. Do something that would normally kill you, in order to activate one of the totems.
4. Notice that the totem of undying in your offhand is used instead of the one in your main hand.