If Alice and Bob use the current version of the Signal app, they can't. You're worried about updates, fair enough, if that's within your threat model you're free to delay your updates for as long as you want, until you've reviewed them and waited enough to be relatively confident that a backdoor would have been found by then. Splitting the signing key into several like you suggest would delay updates for everyone while bringing negligible benefits to a tiny minority of Signal users.
The Signal protocol[0] is used in Signal[1].
[0]: https://en.wikipedia.org/wiki/Signal_Protocol [1]: https://en.wikipedia.org/wiki/Signal_(software)
> they can read our messages if they want to
If Alice and Bob use the current version of the Signal app, they can't. You're worried about updates, fair enough, if that's within your threat model you're free to delay your updates for as long as you want, until you've reviewed them and waited enough to be relatively confident that a backdoor would have been found by then. Splitting the signing key into several like you suggest would delay updates for everyone while bringing negligible benefits to a tiny minority of Signal users.