iMessage was/is capable of registering a user with mulltiple phone numbers.
I know it, because I did it in the past, having always three numbers registered, when travelling even more; that of course in addition to probably a dozen or so e-mail addresses, all with one AppleID/iMessage account.
This worked fine for sending and receiving messages on multiple iPhones, iPads, Macs regardless with phone number or e-mail was used to send a message to. When initiating a new conversation, or course the one selected in the preferences was used.
Now enter dual-SIM, dual-standby iPhones: they should behave like two phones in one case, and certainly, if a single-SIM phone has no problem sending and receiving iMessages to/from multiple phone numbers, a dual-SIM, dual-standby phone should have LESS of an issue doing so.
But welcome to iOS 12.1.x: When going to the Messages section, you MUST choose ONE of the two active lines. Below a text:
"Messages & FaceTime will use selected phone numbers as your ID. Only ONE [emphasis added] cab be refistered in iMessage. Your second number won't be iMessage capable."
I likely can bypass this, by using one carrier's option to have a second SIM with the same phone number, and putting that into an old iPhone, and registering with iMessage like that, but this is bogus, and shouldn't be required, given that iMessage has handled multiple registered phone numbers for years now.
