Probably not the best idea even if your need more than one of them to spend.
If your going to the trouble of using Multi-Sig then why keep the keys online?
Seems pretty stupid.
Im only talking about keeping the Zpubs online, you can't spend with just Zpubs. you also need the private keys which of course would not be stored online. Im curious if there are risk involved with storing public keys(Zpubs in this example) online.
You already got the correct answer from BitMaxz: zpub is a public key; it allows to see your addresses, but can't spend your funds.
It's a master public key if someone has access to this then they can only see all of your addresses on that zpub but they can't make any transaction.