Hi - I'm not sure you can actually do this - as one compensates for the other.
Onboard graphics is supplied as a generic on most PC's nowadays, and it is up to the end user to replace these with a new card.
How old is your machine? Most new boards and BIOS's pick up an AGP card and shut off the onboard display - mine does this, and its less than four months old.
As for two graphics cards i.e. the card and onboard - I don't recommend this, it will cause problems in Windows and make life awkward for you. That's why the onboard is suspended with a new card.
You might want to consider buying an adapter that allows you to have more than one monitor from your graphics card. I am not sure of this, so you may want to check, a KVM switch may be the answer - I AM NOT SURE OF THIS...
Final point, when you install any new card into a PC you remove the old one and any drivers to avoid conflicts. It's the same scenario with your graphics card.