You can group a One and Arc together to play the same audio at the same time.

Read more here about how to group speakers:

https://support.sonos.com/en-us/article/group-and-ungroup-rooms

NOTE: When playing TV audio on the Arc, if you group the One with the Arc, you will experience a slight audio delay from the One. Streaming music will be in sync though.

Note it won't be a stereo pair, though. To create a stereo pair, both speakers have to be the same model.