You are talking about a hundredth of a degree which is about 1.1km. The resolution of the global models, ECMWF model and GFS, is 9km and 22 km respectively. Limited area models like ICON-EU and NAM offer about 5km resolution. There is only AROME which has an actual resolution of 1.3km.
So I wonder why you want 2 different favorites within 1km of each other while weather models show only one grid point on the same distance.
I have verified that beyond 1km it is possible to save 2 favorites (at 47° latitude, but this may vary depending on the latitude). Try zooming in the map to max., to place favorites clicking the map.