I'm in a poly relationship with two partners. One of them is my Dom and the other is vanilla. They both know about each other. Is it common for people to have kink with one partner and not the other?

Polyamory allows people to have different types of relationships with different people, and there is no requirement that every partnership be identical. Having a D/s dynamic with one partner and a straightforwardly vanilla connection with another is a recognisable and functional arrangement that many people sustain comfortably.

The main things that make this work are transparency and clear agreements. Both partners knowing about each other is already a crucial foundation you have in place. What can sometimes need additional attention is making sure neither partner feels like a lesser version of a relationship the other one has, or that the kink dynamic creates a hierarchy where the vanilla partner ends up feeling secondary. That is a communication issue rather than a structural one.

Your vanilla partner may be curious about what your D/s relationship involves, or they may actively prefer not to know the details. Either is workable with honest discussion about what level of information-sharing feels right for everyone. Some vanilla partners are completely comfortable coexisting with a kink dynamic they are not part of; others have feelings that need to be worked through.

The D/s partner may also have thoughts or feelings about the vanilla relationship. Structure, agreements, and regular check-ins among everyone involved tend to be more useful than trying to build a perfectly stable system once and never revisit it.

The arrangement you are describing is common enough to have its own language in polyamorous communities, and the resources on 'kitchen table polyamory' and 'parallel polyamory' are worth reading for the coexistence dynamics specifically.