Bike 1:
Speed
Avg. speed
Distance | Time of Day
Cadence | HR
Bike 2:
Current HR
Current Cadence
Elevation | Grade %
HR Zone | Last Lap Speed (5 mile laps)
Far and away the info I use most often is the cadence and HR. I have a routine that just before a climb I will switch to Bike 2 so that the HR, Cadence is most prominent, and so I can glance at the grade. If I see grades >15% and my HR over 182 I know I am hurting and it is just a matter of seconds before I have to get off the bike and walk for not being able to turn the pedals anymore (or fall over from not clipping out).
When I am on flats or downhills I will switch to Bike 1 so that Speed is most prominent.