This is how I always express this.

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Note added at 53 mins (2017-04-21 11:59:58 GMT)
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A few references (though you can just Google the phrase and see myriad baby/parenting posts and forums with this phrase):

http://www.livescience.com/35354-pregnancy-interval-children...
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4332936/

A few notes: Don't write it as close-in-age siblings- not common. I saw it as "cluster babies" in one Washington Post article- this wording is not standard and would not be widely understood, if at all. If you refer to one sibling in reference to the area, write "close in age to" not "with."

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Note added at 1 hr (2017-04-21 12:22:31 GMT)
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This is the standard way of expressing this phrase (or children close in age, more generally). After using it the first time, though, I think it's fine to use close siblings or, preferably, just siblings in the rest of the text.

Since your text is about strollers (thanks for providing the context), just "siblings" or "sibling stroller" a secas is what you want if you need to refer to the two riders. But the general name of the product is a "double stroller," at least in the US.

-Top Double Strollers for Siblings & Twins
-Children who are close in age (less than 2 years) seem to do well in almost any double stroller, while children who are 2-4 years apart often do better ...

Ultimately, it's obvious that only two very small children can ride in a stroller; they have to be close in age. But for the purposes of a glossary or standard term, I think "siblings close in age" is most accurate.



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Note added at 1 hr (2017-04-21 12:27:45 GMT)
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Double pram/pushchair/buggy for other countries, I suppose.