[2021] I think this is how the double regions cards work. Please correct me if I'm wrong.

to clarify rules: multi region champs count as both regions when being targeted or looked at by card effects. for deckbuilding purposes, they only count as the regions of which you have other cards.

if you put teemo in a deck, then add a bandle city card, then add a noxus card, you're fine. if you try to add a piltover card your deck would be illegal. you can add bandle city cards and noxus cards as you like including cards that are multi region as long as one of the regions is noxus or bandle city.

if you put a teemo in a deck, then add a p/z card, then add a noxus card, you're fine. if you add a bandle city card, your deck is invalid. you can add any number of p/z and noxus cards, iuncluding multi region cards that have noxus or p/z icons, regardless of other icons.