If you have a look at the Shimano docs on this what you will find is that it works similar to other dual sided PM’s (eg Vector or Powertap p1 etc) but with one important difference. With most dual sided PMs one crank or pedal is the slave (usually the left side) and it transmits to the master (usually the right side). This is then combined and sent as a single data stream to the head unit that will have the following day items in it: total power, Power balance l/r, torque effectiveness l/r. Shimano has direct linked via a wire the two sides together so it eliminates one wireless transmission thus making it less likely to have errors (eg my old vectors use to loose the L side sometimes). On the Shimano the L and R data for the above data items are combined and sent as a single stream. If your srm head unit can display l/r balance and l/r torque effectiveness then you should get that data. I never look at it while riding.
I just upgraded to the Shimano from Stages so that I would have the complete picture of my power rather than the assumption of L=R. I am actually 48/52 according to the Shimano (overall ride average). Thing I notice from the Shimano is very stable cadence due to magnet that works down to very low cadence. The Stages was flaky below 32 rpm due to its sample rate (64 samples/sec). If you understand the maths behind this you will know why 32 is the limit for the Stages.
I also see much faster pickup of power changes on the Shimano also and I would think this is due to much more accurate cadence detection.
DM says R9100-P streams
- Left/right power balance
- Pedal smoothness
- Torque efficiency
- Battery level information
PC8 displays and logs the first three, not sure about the rest.
I see average 53/47% balance on my first ride.
All the Dual-Leg Powermeters when using ANT+ "Standard Protocol" will have just one unit streaming data using ANT+, when using Pioneer, you pair the left sensor to the right sensor and the right sensor is the one sending the data using ANT+, this is exactly what Shimano does, the difference in Pioneer is that when using their Protocol, they are able to send two different streams to their computer, these, you have two independent sensors.
To clarify it, when using the "Pedaling Monitor" mode in Pioneer, you will see two units (Left and Right) in their computer, when using "Dual ANT Power" mode you will see just one unit and when using "Single ANT Power" you will see two units but each of them is actually just doubling (unless you changed it when switching the mode) the data (i.e: one legged power meter).
I'm not sure how Garmin does it, but i can assume the same.