What is Adstipulator?

Legal Definition
In Roman law. An accessory party to a promise, who received the same promise as his principal did, and could equally receive and exact payment; or he only stipulated for a part of that for which the princtpal stipulated, and then his rights were coextensive with the amount of his own stipulation. Sandars, Just. Inst. (5th Ed.) 348.
-- Black's Law Dictionary