Short answer: it is not an option for your needs.
It is very very difficult - basically, you want to use the agp card on mobo2 with the bus and cpu advantages of mobo1. Unfortunately, the bus HW on mobo2 is controlled by the chipsets on mobo2 including the slow clock and cpu and so on.
The obvious approach - network the two boards - is not viable since this just involves running the agp card on mobo2 at mobo2 speed and sending the result at ethernet speeds to mobo1.
That leaves somehow connecting the agp slot and supporting HW to mobo1 - which would involve a lot of delicate cutting and soldering ... probably a custom circuit board too. Probably more expensive than a second-hand pcie card unless you happen to have a high-tech electronics lab handy.
BUT: maybe someone has already done it for you ... what about a pcie to agp converter?