Brother of The Benz, p Tompkins
The diameter differences of the two pulley would only effect the rotation speed.
Relative to one vs two belt drive many times depends upon the amount of belt contact there is with the pully. With fewer degress of contact, two belts whereas more degrees of contact, one belt. There is no builtin safety feature for the P/S.
Using only one of the two belts, will the original belt align to one of the groves of the used pump. If yes, then just use one grove of the two grove pulley.
If the used pump was a Vickers and the original the same, the swapping of the pullies would solve your concern; likewise with a ZF. It is possible the pulley from one might fit the shaft of the other.
The pulley is held on the shaft by an end nut. The pulley aligns and drives with a woodruff key.
Disassemble the pully section of both pumps to see where you stand.
Is there some help out there?
Happy Trails Beep Beep from The Spiderman in Houston!!!