hmmm, there isn't anything specifically built in for something like that, but, I think it would work anyways. Example:

You can add as many envelopes as you want, and as many accounts as you want. So, you add all of the joint accounts and all of the personal accounts.

The accounts page will show the individual balances of each account as well as a combined total.

The envelopes will show the balance of each envelope as well as the combined total.

It'll just be up to you to put the income and transactions from each account into the envelopes you want them to go into.

I think this works, assuming I'm understanding your question correctly.