Definately my field then:)
Firstly hello and welcome. I'm Fiona or Fee. Mummy to 3 year old Ella and 1 year old twins Katie and Abbie.
When I found out I was having twins, I needed a new pram. I had a Neo concorde with Ella and it was good (had it's failings but what ones don't)
I wanted a side by side pram that allowed both girls to lie completely flat and preferrably facing me. Eventually I settled for a pram from a Scandavian company - it was called a Boston Duet. It did the job for the newborn twins but not great as they got bigger.
Your situation is you need a side for a baby and a toddler. Having been at the twin club for a while now, I have been in contact with many prams. A lot of people go with the Icandy Peach (one in front and one behind) or Jane tandem, or Gracco sport Duo. But these all have one in front and one behind.
We have a cosatto me to you stroller and I love it!!! You can get lovely designs on them, - mine is bright pink with "Sugar and Spice and All things Nice" written on it. Best of all it fits through all doors easily, some of the side by sides are much much bigger and dont fit.
Others swear by Mountain Duet or the nipper 360.
Personally though with your eldest being that age I would consider a buggy board on the back of your pram to let her stand on when tired. We had a buggy board for Ella for when the twins were born and what a lifesaver. She could jump onto the board when tired and walk when not. If you do go down the twin buggy I would go with a 2nd hand from ebay or the like. Chances are you wont use it too long so not point spending a fortune on it. I got my cosatto buggy from Ebay for £40