Using 5400 as output slave (all rendering will be done on 7850) is impossible in your case, because:
1 Proprietary drivers doesn't support DMA-BUF due to legal issues (long story). Catalyst doesn't support it too.
2 Using offloading/output slaves is possible with FOSS drivers r600g (for 5400) and radeonsi (7850) but radeonsi is not quite ready.
So maybe you will use (mini)DisplayPort outputs that your 7850 already have? (Sure, in this case you will need DisplayPort to HDMI/DVI adapters.)