Looking a MGB's, it looks like they started with the 1969 model year. All the ones up to 1968 have two wipers.

https://www.hemmings.com/classifieds/?0=0&makeFacet=MG&modelFacet[]=B&adtypeFacet=- Vehicles for Sale&page_size=60&sort=year_asc

All the XKE's I checked had them