I’m going to detail this as I interacted with it. My WhatsApp is on my secondary line, the one that I don’t use a lot. But I needed to have WhatsApp on the phone that will be my daily driver (a Galaxy A5 2015), the other is an Alcatel Idol 3 4.7 inch, feels tiny for my daily typing.
So I just uninstalled WhatsApp on the Idol 3 and installed it on the Galaxy A5. Note the line that my WhatsApp is on is on the Idol 3. Proceeded to add the secondary number when setting up WhatsApp, the text was of course sent to the other phone. I fed it manually on the new installation and we were good to go.
The rest is the usuals, restoring from drive and I have WhatsApp on the other phone.