On my tri monitor setup I have an FX5200 AGP card with 2 monitors hooked to it (middle and right monitors), and a random Matrox PCI card for the other monitor (left side monitor). Plus as I already said, I did the PCI and onboard together before...
Just for the heck of it I tried opening Google Earth on all 3 monitors, it only showed up on 2 of them (the monitors hooked to the FX5200), so it isn't the most stable way to go, but I don't have another AGP slot and an extra FX5200 either...