That may be the vendor's requirement because it wants to port all the tax reporting info (the tax owner's SSN, primarily) over to the new account, along with the W-9 info. Otherwise, there should be no reason that the non-tax owner should not be able to open a new account for which they are the tax-owner.
In other words, it may just be a problem with the moving parts in an online opening for current joint owners, since there should not be a problem at all if it were done in person.
Could you open the account with no deposit and fund it after the fact with a manual transfer approved by the opening owner?
John S. Burnett
Fighting for Compliance since 1976
Bankers' Threads User #8