I would say don't bother.
I was sure I needed one as was still using a pushchair a lot for W as she was happy to go in it and was easy to contain her when in shops etc and if she got tired walking.
I bought one second hand .
B is now five weeks old and I've got the double on eBay to sell as don't like using it!
I like the idea of a double for the reasons above, but it was much more awkward and heavy pushing two and so I found myself always using the single-
Either B in a sling and W in pushchair, or B in pushchair and W walking.
We live in a small town where everything is within ten mins walk but it's very hilly. So I just have to be mindful now that if W is walking I don't go as far. If I need stuff from the supermarket I tend to ask H to get it on his way back from work, and if I was going out of town it would be at a weekend when H was with me to help.
I debated keeping the double for incase I needed it occasionally, but decided its silly to have something taking up space and that I will hopefully get the £150 back I paid for it just on the off chance I might use it once in a blue moon.
I really wouldn't bother with a double. A single is much easier if you half a toddler who is half decent at walking