hi liisa
i would back up what others have said here, in that, my dd1 was 2.7 when dd2 was born i used the pram from first daughter this was mamas and papas 3 in 1, with buggy board, and a framed raincover (really big) was big enough to use with dd1 standing on buggy board but also under the raincover (ie protected from rain/snow) u cant get these raincovers now but could get on ebay for about £10, if thats any use
if you feel you really need a double u could always buy the above and buy a second hand duette chassis this allows you to use the 3in 1 combo u already purchased in tandem style but you can arrange the two units in any order, ie seat unit at front and carrycot at back, or car seat at front and seat at back facing away from you, or towards you.
i used one for a few weeks as a friend lent me one, but it would have been the only double i would have ever bought, due to a wrist injury i couldnt use the normal tandem style as heavist child went at front and i couldnt lift it.
this combo meant i could have carrycot at front and dd1 at the back and this was also the only tandem i found where i could fully recline the back rest for older child
certainly worth thinking about, its a great buy if you contiued using the duette and lo grows out of carrycot you can buy a second seat unit, and use two seat units, both can face you, both can face front, they can face each other, or back to back so very versitile
HTH