@tendertouch'Irish twins' was the answer I was failing to recall. (Thank you everyone who suggested it)
The top Google result says:
The term "Irish twins" refers to one mother having two children who were born 12 months or less apart. It originated in the 1800s as a way to poke fun at Irish Catholic immigrant families who didn't have access to birth control.
SOL has 33 stories containing 'irish twins', and the majority appear to use the idiomatic meaning. Michael Loucks, Jay Cantrell and aroslav all have two entries and even G Younger has one - all stories I'm likely to have read, although their definitions may have been widened to 15 months. So I don't know which story I remember it from :-(
AJ