Be careful as lightweight but sturdy enough for two kids = convertible is not really going together well. There are lighter double strollers for two but they are either very simple (better for toddlers, like, umbrella buggies) / small in the seats (the seats will be smaller in any case if the stroller can fit two kids anyway), and/or not really sturdy.
I would definitely suggest the Bugaboo Donnkey as the lightest option. It is simply well made and truly universal, and as opposed to one-behind-the-other tandems, the push is 1000% better. From tandems, I would look into iCandy because of the sturdiness, and also the Uppababy Vista as the basket is great and the frame really holds well.
Personally, though, I am not sure I would go immediately into a single-to-double. You don't know how the first child will be, if you will get pregnant quickly, how your life will be. It is not hard to sell a single stroller and get a double, however to have a bulkier one with a smaller seat and eventually not having that second baby seems like an overkill.
That is of course my personal opinion based on such experiences (e.g., I had my sons very close after one another, and my first hated strollers, so a double, yes we tried, was an incredibly wasted investment).