GTA can't see 2 Gb, because data is duplicated in Crossfire. 1 Gb is just not enough for high settings due to quality of assets in PC and PS4/Xbox One ports. Not to mention that older AMD cards had some problems with data streaming in this game. Reduce your settings.

I've got it on low settings and it's quite happy at 60fps with 1.7GB VRAM usage in afterburner. Well, it's happy except for the major stuttering. I've run it on low settings on 1GB cards before (GTX 560, 645) as part of benchmarks and its performed fine at mimimum without the stuttering. It must be crossfire related of this particular gen of ATi card related.

Yeah I think you're right about it being a duplication thing as Bioshock infinite runs flawlessly at 75fps with nearly max settings and 90% utilisation-ish on both GPUs, but shows 1.8GB of VRAM usage in MSI afterburner.

there was some techie on yt ( i have no experience/knowledge myself) who stated that stuttering was performance related on ati solution so the cpu is not fast enough to coordinate the work of 2 cards at the same time; im not sure if this is helpful, but maybe the cards can be downclocked to verify

Catalyst drivers (some versions at least) sums the total VRAM capacity as well as the RAM usage values when running CF. So if it says 2 GB total and 1.7 GB used, then in reality you have only 850 MB used (which sound about right for low settings). Also I remember GTA 5 runs worse with two HD 4800 cards than one. I had older drivers, but I guess it might be the case for all of them. So try to disable CF, not much else to be done I think.

Too old for GTA 5 with HD4890 cards, and GTA 5 is demanding game. Think about more newer video cards in CF or SLI. Try Directx 10.1 if you must.
Needs ram to be more than 16GB, CPU should be Ivy bridge or Haswell or later.

Recommanded is HD7870 or slimilar age of Nvidia like GTX 680 or later.

Cheers,