I like everything about my Monster, with the exception of the fact that it's miserably uncomfortable when my wife rides with me.  It's painful for me and for her.  I knew going in that the Monster is not ideal for two-up riding, but was hopeful that with the changes since my 09 Monster, the bike might be more comfortable.

My question for all you mod guys, are there any mods I can consider that would make riding two-up on the Monster more enjoyable, even if it's for shorter rides?

I look forward to and appreciate your feedback.

-Drawbridge

I think the question is, what specifically is painful?

cycle-ergo.com shows a more upright position on the M1200S compared to the 09.

Lowering the handlebars would make you lean more, and make it easier for your wife to reach the tank.

Triboseat (sticky cover for the pillion)

'Love handles' (belt around your waist with handles for the pillion to grab)

http://www.jesterstrickbits.co.uk/products/Pillion-Tank-Grab-Handle-for-most-makes.html

Sargent seat or Airhawk seat pads. Can recommend them both.

My two up riding comfort comes from my wife riding correctly
If she doesn't pay attention and slams up against me on stopping it socks
If she doesn't sit upright and lean in the 4 correct directions at the same time it affects both of us.
But I do ride smoother with her on. I use the touring mode, prepare for longer stopping distances slower corners less harsh acceleration
We've  ridden 09 696, 98 st2, 16 821 and the 821 is probably as comfortable as the st2.

Quality rear shock w adjustable preload.
Makes bike behave well w/wo pillion.

Chicks love Airhawks.