Hi Karia. Congratulations! My older two are just 15 months apart, and I have to say that it's the most wonderful thing in the world. They are best friends and play together all the time. Of course they get in arguments as well, but that's part of life.
As for the double stroller, I used the Graco DuoGlider when my second was born. It lasted a good two years. I walked that thing to death, too, using it almost daily to walk into town, the park, the library, everywhere. It worked great with the Graco snugride. The only real problem I had was that I did not find that it was particularly easy to push. It wasn't horrible, but I wouldn't call it easy.
After my third was born, I bought two new strollers that I think you should consider. One is a Valco Runabout. The double stroller is huge and cumbersome, but if you think you'll have a third soon, it's totally worth it because you can buy a toddler seat to go on top of it. Check out valcobaby.com. If you believe you're going to stick with 2 kids, you could get the Valco single stroller and get the toddler seat for the top. My kids LOVE this stroller. And it is SO easy to push. It's amazing. I think it's probably big to fold, though. I don't mess with folding it as I only use it from the house to walk into town, the library, the park, all that.
I also bought a Joovy sit-and-stand that I keep in the car for trips to the mall, grocery store, etc. It seats the baby and my middle child. My oldest can walk for those shorter trips. The sit-and-stand is GREAT. It pushes easily and folds easily. And my Graco snug-ride car seat fits easily into it. The only thing I'm unsure of is if it will work for a 14-month old. I think my 15-month old would have been ok most of the time. If your older child sleeps in the stroller sometimes, this definitely wouldn't work as the sitting part for the older child is not something you'd snuggle into. I'd suggest going to Babies R Us or Right Start to check out the stroller and have your older child ride in the back part to see how she'd do.
On a side note, the first few months of having two such little ones can be difficult in terms of lack of sleep and managing two who don't listen so well, but I promise you, it gets so much better. I really want my third child to have a brother or sister that is as close in age as well. I think it's a great gift. Congratulations, again.