I think the sportster is fine two up, if you take the trouble to set it up for that.
Stock, the bike is poor solo in my book.

The wife and I have been two up on various bikes, a 1969 Triumph Daytona (500cc 4 speed!), a 1979 Triumph Bonneville, a 2005 Triumph Bonneville, the 2009 1200 low, and all the other bikes were better solo or two up stock.
The older Triumphs were the most comfortable.

The low had a useless seat, and a very bad suspension, and even solo it was good for about 1/2 hour.

With a sundown seat, lower handlebars and a better suspension, its at least as good as the other bikes in comfort.
The wife and I have been out on some 3 hour rides, and we both get stiff, but I think we would on any bike.

Last week, I did 800 miles in 3 days, two 300 mile days, and a 200 mile day, all back roads, so lots of time on the bike, 12 hours a day about.

I was very stiff and sore, because I am not used to the bike, and have some other comfort things to try (hiway pegs).
I have only had the bike about 6 weeks (2600 miles on it now).

I would much rather have a fun sportster then a comfortable boat.
I will trade off some room and comfort for the raw fun of passing slow pokes on twisty roads in no passing zones.
More fun then sex!

Brett

 

Even with 450 pounds on it, my 883 rides just fine with stock 99 shocks and the mustang seat for two. I would rather be on my Sporty than be in my recliner. But that is just me.

 

We decided to move the shocks down two notches, and it made a major difference! Every bump broke our spines, I was surprised how that little adjustment made such a big difference!

 

the railroad tracks in town use to be awful to go over on our heritage. it use to jar the fire out of both of us. we rode over them the other day for the first time and my wife said the sporty was smoother and didnt jar as bad. i am 228 and she is 120 +or- a pound or two.