Tutorials on replacing Dropbear with OpenSSH would have you disable and stop Dropbear once you have OpenSSH up and running. An example would be:
Question: Would it be "machine-wise" okay to have both Dropbear and OpenSSH running, listening on different ports?
"Machine-wise" means basically whether the two SSH apps would run into conflict. (I don't quite know how to generalize from that.)
I'm setting aside administration or security issues (more work, less secure). Thanks.
A BIT MORE CONTEXT
My plan is to make custom images with both SSH apps running but, on flashing, disable and stop Dropbear by hand. Password authentication is to be allowed for Dropbear, but not OpenSSH (which is to be key authentication only).
This way, if anything happens to the router and I have to factory-reset it, it would come back to life in a form that is accessible to SSH with a password. If in the mean time OpenSSH client key authorized by the router has disappeared, I can get the router to accept a new key (after SSH-ing into Dropbear with a password).
I even considered so creating the image that, on flashing, the router will have no password and allow free access. But that would mean I really must remember to close things down right away.
Yes, an alternative would be to let OpenSSH start life (on flashing) with password authentication allowed. But my plan is that OpenSSH will be open to WAN, but Dropbear only to LAN.
Oh yes, I know that Wireguard is better, and I will get there eventually.
Anyway, my purpose in this post is not have this overall plan sanctioned, but to have the technical question answered.