It's more every city/arena has an emergency backup. Each team has to have someone on call who can get to the arena in the event either team has a last minute injury to one of their goaltenders. The backup for Philly has "played" for both the Flyers & Hawks.
I don't think they even get money [1]. They do get to keep the jersey though!
[1]: https://www.secondcityhockey.com/2016/12/3/13829264/eric-sem...