OUrlux
I bought a double stroller (tandem pram) as Shelby was 21 months when Sam was born. She loves walking also, but there are times when you want to run in and out and having a toddler trying to do their own thing can be hard.
To be honest I dont use the pram very often, I prefer to have Sam in the sling (so does he) and Shelby goes in the stroller or walks.

The only to thing to think of, is when bubs gets to heavy for you to have in a sling, there may still be times when Lucas wants to sit down as well, I mean he will only be 2 1/2 by then and his little legs will still get tired if you are walking around the shops, he may have had a bad night, or not feeling the best, he may also feel the need to go in the stroller because bubs has taken his place in it ( we also have this problem with Shelby). Have you thought of how you might cope then? Maybe try with a stroller and sling for now, and then purchase a cheap side by side stroller that is suitable for 6 months plus down the track.