Like squonk says - tough one!
My DD was 3yr3mn when her DB arrived, and even though she more or less abandoned her pushchair at 18 months I found she regressed quite a bit and was jealous of him in the pram or pushchair - albeit a fab sister to him.
Luckily we had her stroller and a couple of times we took both, but obviously only possible when both me and DH were together.
Like you say if you take them out on your own you have no way of controlling them both together without having them in a double buggy.
But in order to get one that accomodates a newborn you are going to have to spend a bit more than if you were getting one for two older infants.
My SIL has 1 year and 11 months between her DD's and she has a one in front one behind contraption, but she say finds it heavy up curbs as the older child is in the front and also very long when negociating corners in shops.
There is another option, I have seen a sort of bike seat ride on that can be attached to a buggy, whould you older child find that more fun than a buggy board?? Also not standing so no complaints of hurting feet.