Ok so I was in your exact situation last summer - my kids are 25 months apart and my original plan was to forgo the double stroller, carry my baby in a sling, and push my toddler in his usual stroller for longer walks. Or put the baby in the stroller and have toddler walk for short walks.
The reality: my baby got really big really fast - like 18 lbs at 4 months! Also, I ended up with some physical issues related to the delivery that caused me to not want to carry a giant heavy baby a lot. Also, it's darn hot in the summer and carrying a baby next to you is even hotter (my baby was born in late June). So I ended up NOT going on many of the longer walks that I wished to go on. Finally, in November when my baby was 4 months old I went and got a Sit N Stand type stroller (actually got a Mia Moda Compagno, which I love). I wish I had gotten this kind of stroller back when my baby was first born. Yes, it's not super light or small, and if you want to put a newborn in you'll be using the carseat adapter (though the front seat of this model, unlike the Joovy and Sit N Stand, reclines quite far), but it's GREAT for long walks. I would not recommend the boogie-board type accessories - they're expensive and get in the way of your feet. Plus, a 2-year old will still want to sit when she gets tired and a stand-board won't allow that.
So ask yourself the question of how and when you'll be using this stroller. Is it for long walks in the neighborhood? If so, I recommend a sit/stand stroller. For in/out of the car? If so, I recommend a cheap umbrella for your toddler and a sling for your baby.