I don't think I would consider a double stroller for children 3 1/2 years apart, personally. My 3 1/2yo definitely still rides in the stroller a decent amount, but my 22 month old is still worn 90% of the time that we are out and none of my kids have really had any desire to sit in a stroller before age 2, so personally I would plan on wearing the baby and hanging on to my single strollers.
I have a phil and teds double, which I love, but I don't thik it would work very well for a 3 1/2 yo and a newborn. It might be a good option once your baby is a little older though and can ride in the toddler seat instead of laying down.
You might look into getting a buggy board for one of your singles, or one of the Peg Peregos that have a little stand on the back for an older child to stand on. A sit and stand type might work too, but when I looked at them (when expecting #3) I fond that the seats were very small and didn't work well for my 3 1/2yo and 20mo.
In general, most doubles are made with twins or closely spaced siblings in mind, so it's hard to find something that will work well for a larger age gap, at least until baby is sitting up.