Hi- I got a tandem double buggy from a neighbour, who felt sorry for me seeing me struggle with my toddler in a single maclaren and baby in a sling. I think she got the tandem for her dd originally, but it was so heavy and bulky dd couldn't lift it into her car. I should add that this was a v.old tandem though, so they might be a lot lighter now... My experience with this double made me decide against tandems though, as I found it hard to get up curbs with the weight of my ds1 in front, and I didn't like the fact that the front seat didn't recline, as ds1 still sleeps during our strolls occasionally.
Now I bought a babydan reykjavik side-by-side on ebay. It came with raincover, no cosytoes, two smallish shopping baskets. It looks a bit like a maclaren, also umbrella folds, but is a more budget buggy: mine cost £79 (+PP 19.95) new, compared to almost £200 for a maclaren twin techno/traveller, wouldn't get a twin rally, as the wheels are v.small which makes it uncomfortable to steer/walk with).
It is a bit on the heav side (14kg), but hen again most doubles are.
If weight is most important to you, you might want to check out the double nipper (3-wheeler). This one weighs 9kg and is one of the lightest. I personally don't have any experience with it tough, so couldn't tell you how easy it is to push etc. I do know that even 2nd hand they are v.expensive.
You might want to go and try some out, incl folding and lifting: if you find one you like you can always go home and look for that model 2nd hand on ebay (?).
Anyway, good luck on your search!