IDK, I think it is plausible they might be joining by IP or deeper connections. I've seen some ads that aren't relevant to me but are for something my partner was looking at on a different device.
It's probably simpler than this - advertiser just runs a campaign with some broad criteria (like income estimate based on IP-geolocated ZIP code) that both people in the same household fit. No surprise the same ad that one person had seen (and remembered) will be eventually shown to the other.