I have a 2017 XL1200T. I have just fitted the extended switch cap. Whilst I had it in bits I noticed that the turn signal switch is in fact 2 switches.

Does anyone know what that switch does exactly please? I couldn't detect anything happening when I pressed it.

 

I placed extensions on my 2011 XL1200L ts switches but not sure what you are referring to in the other switch. Pics may help.

 

Total Guess:
Emergency Flashers?

 

You can see in the picture I am pushing on the left hand side of the switch. Underneath the cap there is two switches. This will potentially only be for 2014 year bikes and newer when the MoCo started using CAN for the switches and not individual wires.

If by emergency flashers you mean hazards with all 4 turn signals flashing then that is the triangle symbol switch above.

 

The microfiche of your bike's handlebar switches only show it as a turn signal
http://partsfinder.onlinemicrofiche.com/ronnies/showmodel.asp?Type=13&make=hdmc&a=1200&b=51&c=0&d= 2017%20XL%201200T%20SUPERLOW%201200T%20(LL)%20TURN %20SIGNALS%20-%20FRONT

What happens when you press that side of the switch?

 

I didn't detect anything happening but didn't play for very long. I will check it out more thoroughly later.

 

Total Guesses
Auxiliary Switch?
Heated Hand Grips?
Three Blink Passing Indicator? <-- Only on Right Side, Self Canceling

 

Okay, I think I have worked it out having put the extended switch on the left side.
There is a single turn signal switch not a left and right, therefore both switches are the same with a left and right actuation.
The stock switch cap can only be pushed in on one side but the extended switch cap can be pushed on both sides.
Pushing the switch on the "wrong" side doesn't do anything and the indicators work as they should.