There would not be a problem if the two trusts simply used the box to store important papers (the trust documents, CDs, deeds, etc)., for which ownership is clear (and which aren't actually assets at all). That would not intermingle assets. But if items of intrinsic value or cash were stored, intermingling would certainly be possible. And that creates a problem, since the bank has no way to know what's in the box.
And there is always the fact that the bank decides to whom it rents the boxes.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8