Yes, you can have multiple keys from different devices active. Try sending a omemo encrypted message from the device you have problems decrypting from, as sometimes it needs such a trigger to sync public keys.

What doesn’t work (by design) is to add new devices and sync old history.

Edit: Also, all chats are encrypted via TLS, omemo just adds additional e2ee which isn’t really needed if you use a trusted server, ideally your own.