1. Desktop with IGD Radeon HD4200 + pcie GeForce 8600GT (and GTS250)
DMA-BUF (DRI-PRIME=1) works, but unfortunately nouveau driver is not very fast.
Also monitor sharing works, I was able to connect 2 monitors on motherboard (radon) and 1 on geforce and I got one extended desktop on 3 monitors.
I tried it few months ago. Here are images (you need to "view full resolution)
2. Laptop with IGD AMD + discrete AMD. I forgot what are actual cards, but DMA-BUF works (i didn't tried with DPM yet). Also poweroffing discrete card for better battery. There is bug when closing lid, when DIS card poweroff - need to restart machine. Didn't have time to investigate yet-