The GPUs do NOT work together when running the Ethereum hashing algorithm. Which is why a mining rig functions just fine using any number of GPUs all connected via PCIe 1x to USB risers. As long as you can get your OS to recognize multiple different GPUs in your system, they can all work on mining, and will be given individual tasks. Note: Getting the OS to recognize all the different cards is the hardest part.
A side note since you mentioned it. Last I checked, maybe a year back or so, the minimum VRAM required to run the Ether hashing algorithm was 4GB. Maybe it's changed, but at the time, that VRAM requirement was only increasing.
I'd suggest finding the Ether mining software you want to use first, then find it's requirements. Then secondly, see if your OS will even recognize, at once, all the GPUs you want to use. Windows, and even off the shelf linux is annoying at best to get multiple different GPUs mining at the same time. May want to look into eth OS http://ethosdistro.com/
Another side note, if you want to run more than 6 GPUs off the same system, you likely need a special motherboard to do so. You could be lucky and the mobo you have has enough PCIe slots and just the right switches in the BIOS to make it work, but I doubt it, unless you have a mining board already.
ADDED INFO EDIT:
If your two GPUs are the ones listed in your profile, then you are correct. Those are not new enough or power efficient enough to turn a profit anymore, unless you get free electricity.