The Savage/S40 has adequate power for carrying double, unless you plan to do long uphill mountain grades at highway speed (think I-70 westbound from Denver). But the passenger seat is not a pleasant place to be, according to my wife. About ten miles was all she wanted. I have tried sitting on it, and I have to agree. If you want to do any serious two-up riding, go look for a bike designed to do that job and don't even fool with the Savage/S40. Go to a dealership - or several dealerships - with your wife in tow. Both of you climb onto whatever bike seems to fit, and sit there for as long as the sales staff will let you. I suggest starting with a Gold Wing, to get an idea of what is possible. Most anything else will be a "downgrade." It is important that both of you sit on the bike at the same time, because the passenger seating position will be different with and without the width of the driver forcing the passenger's legs to spread apart (yeah, I know that sounds kinky).

Edited to add: For two-up riding, don't even look at sportbikes. Most cruisers have poor passenger seats. If the passenger seat isn't as wide as the driver's seat, she won't like it. If the passenger seat slopes rearward, she will feel very insecure on any sort of acceleration. Look to see where the passenger footpegs are located. Most modern bikes do not have centerstands, so you will have to convince the sales staff to hold the bike upright while the two of you sit on it - with your feet on the pegs. Or bring a big, strong, willing friend. I know these comments seem rather negative, but there is a lot of truth in the saying "If Mama ain't happy, ain't nobody happy."

How much experience do you have hauling anyone? How much experience does she have as a passenger? A passenger who doesnt understand what looking around a car to see whats coming can do to you is a dangerous thing. My daughter put me in oncoming, thankfully, the road was empty.

+1 on the knocking helmets. Both my wife and I are small ( Im' 5'9" 145, she's 5'2" 120). We ride around town 2-up but that's about it. Plus, whenever she moves around...I feel it. The savage just barely works for us.
That being said, she loves to go for little rides on it. I've tried to talk her into getting a small bike of her own but she doesn't want to have to think that much...
Also, think about the responsibilty of someone else due to your actions...I ride my kids around in the neighborhood (with a helmet on them) but not the major roads...too scary!

I'm not sayin' it doesn't have the power, just that it's not the most  comfortable. Remember, the question was thrown out there by a new rider...

haha no worries.  seeing me on the savage with my various women is like watching two hippos fighting over a twinkie on a Big Wheel.  i generally have clown music in my head when i ride...yeah i got issues

ah, forgot to mention, if you do double up, take a practice ride around the block first. even small girls make the brakes on the savage feel horribly inadequate. long time to stop, wider turns (grind pegs real easy with the extra weight), and harder to stabilize, especially if she likes to fidget. so be sure to get used to all those changes before you go flying down some back country road at 85, laying her down with two feels a hell of a lot worse than laying her down with one  

Helmet Knocking?  oh thats right you guys live in oppressive states that have helmet laws    in NH its your god given right to be an idiot.