After a lot of trial and error and reading of wikis and forum posts, I've got three screens running out of two GPUs with full acceleration throughout.
I'm running Arch Linux on a Gigabyte H81M-DS2V motherboard with an Intel Core i7-4790 featuring Intel Xeon E3-1200 v3/4th Gen Core Processor Integrated Graphics.
Plugged in to the PCIe slot is an AMD/ATI RV710 [Radeon HD 4350/4550] graphics card.
I have xf86-video-ati and xf86-video-intel installed....