I DID go to the movies with DS when he was about 5 months old...he was in the sling the whole time. I lie, except for the 10 minutes I took him out to feed...no-one knew he was there and I got to watch a WHOLE film! And it wasn't even a baby session. I haven't been back since with him cos he got mobile fairly quickly thereafter
Really, I think you could definitely get away with no double pram, Ourlux. Even a 6 month old baby can feel very light on in the right sling. With DS I had him in the HAB for about that long, then switched to the Ergo. In the HAB I was starting to feel it, as he was in and out all the time (of the car, mainly and I needed something a bit more 'snap on'). Once we got the Ergo, it was like he was weightless again!
By two and a half, even if DS doesn't want to be walking everywhere, you might be able to get him a 'chariot' attachment for the pram and an extension handle. That's a bit of messing around, but cheaper than a double pram and far easier to manage, I reckon (you could probably get the chariot and pram onto a bus much easier! It's good to think of these possibilities, even if you have a car you currently rely on...my car recently broke down...my VERY reliable car!).
If your back weren't so bad (and if my hip weren't so problematic...it's getting better slowly), you could even have a soft sling for the baby and an Ergo to have DS on your back. Really, though, I don't know many 2 and a half year olds in my circle of friends who get around being pushed, so you should be fine by then by just using the stroller in the meantime