I have a 2 year old and a 10 month old and I live on a farm so I needed something really robust and able to deal with uneven surfaces etc. I absolutely LOVE my Out n' About Duo. I have gone through several strollers to find one as good and for a reasonable price. (I managed to get mine 2nd hand and in great condition for £100 from Gumtree!) The kids will be sitting side by side and the stroller fits through all doors.
If you are looking for a more tandem style stroller and don't need one that is so robust (ie if you are just using it to go to the store, park etc) then I would recommend the Graco Studio Tandem. Again this is really affordable and a great buggy but it is just not so good on uneven surfaces. I used this stroller when my daughter was first born and before we moved to here on the farm and it is a really good stroller with loads of storage space underneath! It pains me to have to sell it now!
I wouldn't recommend the more expensive strollers (e.g. Phil and Ted) as I have a few friends that have one of these and they wish they had bought my Out n' About instead!
Also if you don't want to buy 2 buggies or a double buggy what about a buggy board? You can attach these to most single buggies and your son can walk for a bit then cruise when he is tiered. I would recommend the Out n' About single in this case obviously as I am just in love with this buggy as you can tell... :D
Hope this helps,