I feel in most occasions it's whoever has the most patience. Some tanks will just stand there with no intention of going tank stance, but eventually someone has to go mt. Generally I'll give it a few seconds and if the other tank seems like they only want to DPS them I'll start things off, otherwise you can try communicating as well and just saying what is your preferred stance. In 24 man there is the risk of anyone in the entire raid pulling prematurely so there is less time to mess about on waiting for someone to decide is tanking. All tanks are viable though and WAR can easily main tank in Deliverance if they know what they are doing.