@Floris-Cyan-Zebra#210486 As easyMoney/easyGreen member you can order as many physical cards of your choosing as you want (with extra costs if you have more than 3 active cards at the same time), and you can have a certain number of online cards, but these are all Mastercards. As the +1 of another bunq user, you can only have one physical card maximum, and no online cards.
If you girlfriend is your +1, she only has access to the Joint account that you both share, and won't be able to have any personal accounts. So there shouldn't be any issues with your girlfriend just using her allotted 1 physical card and ordering a Maestro card, except in case she also needs a Mastercard (for example for traveling). In that case, you can indeed put any of your cards into her iPhone/Watch's Wallet app, and that would also work fine.
However, as I mentioned above, bunq cards are always personal. There are several stipulations in the terms that don't allow you giving away any of your cards to other people, be that in physical form or by putting it on their phone. (To be exact: Chapter 36 of the terms of service, and chapter 11 of the card rules.) So at least I would keep that in mind.