This is a multinote tremolo. Those last one beat, so you can input a A and a F as 8th notes, then a G and a F, select them, shift-R (repeats popover) and write 22 (two slashes on multinote tremolo).
Marc, I’m not sure if you can get that notation with eight notes. With quarter notes, and this setting under Engraving Options->Tremolos? you can, but they will last a half bar.
Ok, you’re probably right, the picture was lacking some context, but yes, it’s probably a duration of two beats and not one, as I initially thought. The command in the popover is the same, but with four quarter notes instead of 8ths.