What's the actual cost here? You have one app open or another, no different to two distinct chatgroups in WhatsApp except for some small extra CPU cost for changing app focus if switching between them.

The only issue I've seen is when I want to add someone who's only WhatsApp to a Signal chat, which has happened maybe twice until they also joined.

Personally I feel good about taking even a small amount of Comms out of Zuckerberg's clutches