An extended public key (xpub) is a master public key created by a corresponding master private key (xpriv). One way to think of an xpub is as a master lock. An xpub generates individual locks to all of the individual keys the xpriv generates.
All individual public keys (pubkeys, or "locks" from our analogy) are derived from the xpub. Pubkeys are used to create individual bitcoin addresses. You can build an address with 1 pubkey (single signature addresses), or you can build an address with 3 pubkeys (multisignature addresses).
When you upload 2 xpubs from 2 hardware wallets to Unchained Capital, this allows for new addresses to be generated for as many multisignature vaults and loans as you would like.