I just took a look, and I don't think that it does support sharing, but then I'm not much good in the BIOS, especially since my owner's manual for the motherboard doesn't have any information about the BIOS in it, other than how to flash it.
The only thing I found that looked as though it related to the video card was the Chipset section of Advanced Settings, where it listed setting for PCIe2 and PCIE4 link ASPM. Don't know what that is for, unless it has to do with setting up Crossfire or SLI. It lists Disabled (current setting), L0s, L1, L0s & L1, L0s downstream, L0s downstream & L1 as alternative settings. I also found where apparently it lets one choose whether to use a PCIe or PCI video card, but none of this looks as though it would be any help.
If sharing can't be setup, then adding more RAM wouldn't do any good, it would just give more space for the system to function, but the video memory would remain a problem. Surely there must be some solution for this that doesn't cost an arm and a leg?