so, to answer the op...
it's possible...
namely the mobo has to support the function, and it's setup, ie.,
onboard is agp, add on is pci...bios supports the function
and hopefully, in random variable X OS, there are no resource conflicts with this enabled...
so, you will have or may have multiple issues to contend with, start with your mobo.
EDIT: edit myself, wrong edit, hehe