I currently have my armrests in the narrowest config, and so they touch in the middle. However, since the armrests are quite wide, I can't easily bring my elbows together and hold it there, since there's nothing keeping them locked in. If they were about half as wide, and still touching in the middle, then it would be perfect. I see the race cups, which sounds like a possible option. But am I correct that the race cups in their narrowest position on the Aeria Ultimate would leave a gap in the middle? I guess the only way to achieve what I want is the reverse of the mounting bracket, which isn't advised and also then I would lose my tilt. The AU's are already super adjustable, so I'm not complaining, just can't hit this particular config it seems.
Had the exact same problem with the cups coming with the AU. I switched to race cups and it is definitely better, although it might still be too wide for you. To give you an idea of how close they come, my BTA fits perfectly in between the two cups with very little space on each side. Also you could invert them and they should come a little bit closer.
On a side note, if anyone is interested I need to part my AU stem (100mm) + hydration (HSF Aeria). Unfortunately, 100mm is still too short for my needs :-(