I would second the double pram too. I had as my car/shopping pram the BJ city mini double and it was pretty light. Otherwise, just stick with a single with skateboard option, and take your sling out with you so that you can swap your poppets around to suit your/their needs. I"m planning on DD3 spending a lot of time in a sling cos I'll be doing lots of in and out of the car with kindy/pp drop offs with DD1 and 2.

You should be able to loosen the extension strap on your bub's car seat every time you put him/her in or out. just make sure you tighten it again before going anywhere. It was one of the things the kidsafe car seat fitter man told me and said that so many people don't seem to be aware that you can do that to make it easier.