It supports linking two Android devices.
And given how in this case Molly could fix it it cannot have been that hard to fix.
"If you wish to use the same phone number for both Molly and Signal, you must register Molly as a linked device. Registering the same number independently on both apps will result in only the most recently registered app staying active, while the other will go offline."
Specifically I'm using Signal as the main device, with Molly as the linked device on 2nd phone.