I originally wrote this out a while ago for a baby-related board I post on, so the names of the buggies I have mentioned are perhaps not the latest, but it should give you something to work from. I have bought, sold and used a few doubles so I hope this gives you something to go on!
Tandems like the Graco are harder to manoeuvre due to their length.. that and the fact the heavier child is over the front wheels, can make them hard work, especially if you aren’t tall yourself..and the older your older child the harder it gets. If you have a smaller age gap, if you mostly go over level ground then they are good.. and some of the Graco tandems take a car seat on the back seat too (but NOT all, ask if you're thinking of using it like this!)
The tandem three wheelers with a swivel front wheel do get round the manoeuvrability issue, BUT are just as hard work to get the front lifted up to go up a kerb.
Side by side doubles are great but I have found the Maclarens other 4 wheeler versions, you don't have so much leverage to lift front wheels up a kerb as you do with the three wheelers.
The adaptability of Phil & Teds is excellent – I now have one of these and totally love it. Because of the age gap with my younger two, its great to be able to change from a single to a double depending on who is with me. I did get this in my Ka without the wheels and handle on, flat in the boot. Also, with the handle on it went in upright – fine if you’ve nothing else to take with you. I know the issue of sloping pavements annoys some people, but TBH its never bothered me… and I can do the school run (kerbs and all) with minimal hassle. I have written a guide for the Phil & Teds too if you are thinking of that as an option.
If you're going to need both in a buggy alot, I'd say the Marco is great for in town - its size makes it great around the shops, and it doesn't have the problems the longer tandems do. Apparently these are hard work if you are less than 5'3 - just have a look at the number being sold on eBay with comments about the sellers height making it hard to push! I think Mothercare do one like this now too. NOTE - I don't beleive these are suitable from newborn though!
If your out and about involves more out & around than shops, then the Instep Nipper, and the Mothercare Urban detour are both fab side by side doubles – the Mothercare one has a swivel front wheel which means its not as compact when folded, but it can turn on the spot, which is great in town and around the shops. The Nipper has a fixed front wheel, but is more suited to farm tracks or country walks... There is also one called a Kingswood Buzz (if you are looking at 2nd hand ones) which is very like the Nipper - just be careful you get the Buzz (it has a bee on the backs of the seats) and not the taller Kingswood three wheeler double which, although it has a swivel front wheel) is heavier and harder to push because of its height.
I have heard lots of good things about the Jane Powertwin, although I'm not sure how compact it is as far as boot space goes, it can take an infant car seat (in fact it can take 2 if you're expecting twins!). Its not a cheap option though
I know the Maclaren Rally Twin I had didn't fit my boot (but then I had a Ka), but they are also good and sturdy. Remember if you have a side by side like this and children of different ages, swop them around. Ie. Don’t make one seat always for one child as this can put undue pressure on one side of the frame and the X’s underneath will give gradually making the side arms not fold into a totally straight line any more. Your local Maclaren service centre can fix this - I know I have paid £40 to have this done once - that was for one X I think!
Finally - Don’t forget, before you set your heart on any double, check it will fit into your car – we changed from an estate car to a saloon when DH changed jobs and the Urban Detour double we had at the time wouldn’t fit though the openings to the boot of the Honda Accord, the BMW, the Audi A4 or the VW Passat! I have a friend who has just bought a Maclaren because it was compact and she stuggles to get that into the boot of her hubbys BMW too… and none of these are small cars!!!