I currently use an iPhone X which replaced my iPhone 6. Can Vsble activate my iPhone 6 with a new phone number so that I can keep my iPhone X separate?
I use an unlocked iPhone 6S currently on Visible so chances are that you can. You'll want to make sure the phone is unlocked though.
If both the phones are unlocked and compatible, you can switch sim between the phones.
You certainly can interchange sims and use whatever phone you want.