This feature doesn’t work on mobile, i have a repeating group set for “full list” and 1 column, with “Allow more than one column when stretched” checked, the RG i set the the width narrow enough so it can accommodate 2 cells on a single row even on mobile size.
On desktop it does show 2 cells on 1 row, but on mobile (Chrome Samsung S6 Edge), it still shows 1 column although it has space for 2.