I think there is some general confusion as to what a wallet is, and how you interface with one. I'll try to be detailed but not too nitty-gritty.
A wallet is really two connected things. First it is a collection of private/public keypairs. In the case of standard addresses (P2PKH) each public/private keypair matches to 2 distinct Bitcoin addresses. This is because you can derive two addresses from the keypair because there is a compressed and uncompressed representation.
In the case of multi-signature the address is constructed based on all of the public keys involved in the transaction. This can create many addresses that represent the same keypairs or combinations thereof.
Based on the comment above you want to run a site that does 2ofN multisignature. Which means you control one of the keys at least, this means that your wallet is a collection of addresses that you have partial control of. At any time you can sum the balance of every address you mutually control with the user and come up with a total balance.
So, the answer is yes, you can contact the user when the balance falls below a certain amount since you have total knowledge of all keys that the user is using since you are party to them. This lets you compute based on the public ledgers information. This is also true for HD wallets / Keys so long as you compute a collection of multisig addresses derived from the HD keys.