I encountered this notation and was wondering what it means.
It's a tremolo. The performer should rapidly alternate between the first two notes and the second two notes for two beats. The notation can be a bit confusing because it looks like it might last twice as long, but both notes of a tremolo are supposed to be written as the full length of the tremolo. Not to be confused with tremolo on orchestral string instruments, or tremolo picking on a guitar, mandolin, etc., although there is some relation.