You have several options: side-by-side, tandem, or sit-and-stand. They all have benefits and bad things.
Tandem: They're long and unwieldy, but you will absolutely be able to use them for kids of different ages. They tend to take up a lot of room in a trunk, but they are the least-costly of double strollers. While the Graco DuoGlider is the standard, it really got that way because they made the most patterns and were really one of the only players in the marketplace. They're monsters and they're common-looking. Kolcraft has one that's more Bugaboo-like called the Contour Options Tandem. Also a monster, but has a variety of seating options: both kids face front, both face back, both face eachother and both face away from eachother. Options, get it? It's *very* reasonably priced and is great looking. I have it. It's the lesser or all the tandem evils, IMO, and comes with an attachment for most infant carriers.
Side-by-side: DO NOT listen to the people that tell you side-by-sides are bad because they're too wide - they're not They fit through standard doorways, and if you're going to be doing down narrow store aisles, are you really going to do with that an infant and a toddler? I didn't think so. There are some *very* expensive side-by-sides on the market - you can spend up to $1,000 on one, but there are very reasonable models - the Zooper Tango is $399 and the Combi is $250-ish; both take an infant carrier. The Aria Twin 60/40 from Peg Perego, which also takes an infant carrier on one side, is a piece of garbage and a waste of money. I was never so disappointed in any stroller as I was in my Aria Twin. Baby Jogger has a side-by-side that's lightweight and $399 and takes an infant carrier - that's the one I recommend. It's narrow, lightweight, great-looking... maybe a tiny bit expensive, but it's a good investment that will take your kids from birth (full recline in addition to infant carrier) to 50 lbs. each. Make sure you check double-stroller weight capacities. Some only go up to 35 lbs each side. My almost-3-year-old is 32 lbs. A stroller that only goes up to 35 lbs is not an option for me.
Personally, I have the Kolcraft Options Tandem for when my daughter was born and my son was less than a year old. I also have a Maclaren Twin Techno for now, and I love, love, love it. It won't take an infant carrier, so it might not be the best for you.
I have done extensive homework on this - feel free to get in touch privately if you have any more questions.