When my youngest was born, my other little boy was 17 1/2 months and I was convinced that I needed a double... big mistake. I used it twice (both times at a mall) and our double jogger was too heavy to push by the time my little guy could sit up in it.
Bottom line is, when you go to the grocery store, cloths shopping, etc... I always used the inside of the basket for the infant carrier and the front for my toddler. unless you spend a lot of time out walking or at the mall, when do you use those?
If you do decide to get one, I would not get a side by side, my neighbor has twins and never used hers because she couldn't get it through doorways, hallways, shopping areas, etc…
Good Luck!
E.