It depends on your goal and your system. If you just want more monitors, you could have an agp 5200 and a pci 9250 in at the same time to give you up to 4 displays(IIRC). They won't "combine" power, they can just let you do several monitors.
If your talking both cards agp, it's not possible, there are no dual agp boards that I know of.
Newer pci-e motherboards can use SLI, which means you can use 2 identical nvidia cards in tandem to increase graphics power, but that's only for nvidia pci-e cards in a Sli motherboard.