We (fboab and me) don't generally need to talk about the running of the bike so much; beyond me shouting "Hole!" or requests like "Stand?" or "Coast" - so you could easily develop some hand/touch signals for those instances - I don't think hearing issues need be an obstacle for tandem riding.
Not on a tandem
[1], but when we cycle, we tend to have simplex audio from me to barakta (direct radio link to her hearing aid, but I can't hear her from behind over wind/car noise), and she responds via hand signals and head movements. That generally works for the sort of communication we need on a bike: "Left up ahead.", "Trim your front mech!", "Watch out for that massive hole!", "Car down wants to turn right.", "Everything okay?", "Slow down a bit", "See you at the top of the hill.", "Oh, FFS, stop antagonising the geese!" etc.
If it gets much more complicated than yes/no answers, we tend to pull over for a chat.
[1] Common sense prevailed when that ICE tandem came up.