If the documents do not say they can act independently, then you are correct in imposing a "two signatures required" approach. If you are going to monitor them, charge for it. If you are not going to monitor them get both parties to sign an agreement that they will not issue any checks without both signatures, but agreeing to hold the bank harmless in their fiduciary capacities and indemnify the bank in their personal capacities if a single signature check is paid.
There's some illogic in that, but there is even more illogic in agreeing to do it without being paid for it.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.