T-deck and iphone to send data

I’m looking at getting a couple of T-Decks, but I also want to be able to use the app on an iPhone whe at home, to send messages. Not at the same time I hasten.

The aim is to connect my elderly father in law with us, but he’s not too good at typing, and I think the t-deck may be difficult, so I want him to be able to send messages by his iPhone, which he is quite good with, and if his phone is flat, via the t-deck. Never by both at the same time.

Is that possible?