Double stroller.

In about a week and a half I will be caring for a 2.5 month old alongside Hadley during the week for our great friends. It's going to be great :) and thankfully we have all of the appropriate gear for a newborn {pack n play for naps, nap nanny for chilling around the house, snugabunny cradle swing, chicco infant carseat base, changing table, etc} but we don't have a stroller for two.
There are so many options out there and you really have to take your specific situation into account when choosing what will be best for you. For us, it's important to function both outside on the sidewalks in nice weather as well as in the mall for all of the nasty Minnesota winter days. Also important to consider is the comfort of both of the kiddos since if they aren't happy the walk or trip will be miserable.
With double strollers you have a choice between tandem (front to back style) and side by side. Tandems are typically really long and side by sides are, well, pretty wide so you really have to pick your poison there. You must also consider the type of infant seat that you have since compatibility is really important for ease of travel with a 0-9 month old. For me, another consideration is definitely the strollers that I have loved as singles since I have loves and definite hates... Love the Uppababy Vista (but Hadley HATES the tiny second seat option and who can blame her it's not that great unless you're just an occasional rider that's small). We also LOVE the Baby Jogger City Mini and of course the Bob Revolution SE. Peg Perego and Britax are not that great by my standards (but as a disclaimer that's just personal preference).
My favorite double options right now are the Baby Jogger City Select ($575), Baby Jogger City Mini GT Double ($669) and the BOB Revolution Duallie ($644). That's about as much as I'll make each month babysitting so not cheapo. :(
The Baby Jogger City Select is the most versatile of the options since it's a tandem that goes from a single to a full size double with stadium style seating and can be configured into 16 seating combos. It folds very easily like all Baby Jogger strollers which is a bonus. The cons are that it can be tough to get over curbs when fully loaded and the rear axel is straight across (so you may kick it in flip flops that's a killer). Being able to go from a single to a double and back to a single with 2 full seats makes this a really great option for our situation.
The Baby Jogger City Mini GT Double is a side by side super version of my beloved travel stroller the City Mini. It has forever-air tires instead of the hard plastic wheels on the original City Mini. The seats accommodate from a newborn (with a head/body snuzzler) to a tall 5 year old so this one likely has the most longevity. The cons are that it's wide and it doesn't convert to a single so if you just have one rider you're still pushing a wide stroller.
The Bob Revolution Duallie is the BEST option for all-terrain outdoors strolls/runs and fairs/festivals. It glides so smoothly over curbs and bumps and almost pushes itself. You can run behind it which you can't do with the others so that's a big plus. The Bob Duallie also has negatives though... it's wider, heavier and longer than the other options for starters. It's pretty much not made for going to stores/ the mall since it's so big. Also, you can't put a baby directly into the seat until they're 6 months at the youngest. While the other options have carseat adapters, the Bob is the only one that you don't have the option to put them directly into the seat until they're bigger.
Other options that I've considered that meet our basic requirements are the Chicco Cortina Together ($299) and the original Baby Jogger City Mini Double ($399). Both of these options are less than ideal for different reasons. The Chicco is a tandem and it is very heavy, doesn't handle bumps or curbs well and is easily the most quickly outgrown. The original Baby Jogger City Mini Double isn't ideal for all-terrain but otherwise it meets all of our requirements. For how expensive it is you would hope that it would fill all of the requirements but no option is perfect so it is still worth mentioning.
There are a lot of positives and a few negatives for each of them so now it's just a matter of processing everything and figuring out what's best in the LONG term (like not just babysitting but for our future kids too).
Since considering all of the above, I've called the store that our Uppababy Vista was purchased from last year and the owner offered a deal that's interesting. He said that he is interested in a "trade" of our Uppababy Vista AND our {wait who are we kidding it's really MY beloved} City Mini for a Baby Jogger City Select with a chicco carseat adapter. This is very interesting but I would be happier if a second seat was included not just the chicco adapter. ...BUT who in the heck else is going to be able to trade for this type of thing?
Jim needs to be consulted since he already thinks I'm certified crazy town as far as strollers go (since we have 1 child and 3 strollers). Yeah... I'm "special" but that's okay.