In terms of something you can just buy and use off the shelf with 2 buttons: doesn't exist. But depending on your budget and how many you actually need, I'm sure something could be created (ie. maybe hard-wired waterproof switches that control a Raspberry Pi that sends out the appropriate IR (or RS232/IP control depending on the TVs you're using). Some remote backend systems could maybe work with similar type of triggers.
Failing that, you could look for assistive products like [Link: assistivetechnologyservices.com]
or [Link: independentliving.com]
(cannot say whether those will actually work, but more the concept of a remote for those with special needs).