There is no way to do this at this time.
We were planning on allowing separate shapes for forward and reverse readings at one point, but I haven't thought about this one in a while. You can actually see evidence of the undone work item in the 'Display Reading Indicator' option, which controls when we show directional arrows with the readings. The default on this option is 'Separated', meaning that direction indicators on readings shapes should be shown when the forward and reverse readings are 'separated'. Of course, this option doesn't make much senses without actually supporting separate readings.
Is anyone else pining for this feature, or other horizontal spacing issues?
PS At least you now have the ability to display value constraints horizontally, which was the other main issue with horizontal space.