The mandolin's two string coursing means you get the sound of unisons in a single "note", but also it also means the plucking of the pick on each string is slightly offset in time. You can use tremolo picking across two guitar strings to get the time offset with the picking, and to some extent you can get unisons.
If you use the
B strings on guitar, you can play unisons and tremolo picking the two strings will get a bit of the two string course sound similar to mandolin. If your guitar has a cut away you will be able to get up to the higher frets, where they are closer together, making the unisons less of a stretch, easier to play.
I think other two string combinations can give a similar effect. Playing in thirds on strings
B or sixth on strings
E with tremolo picking is nice and reminds me of mandolin.
Playing the high
E string open as a drone while playing some melody on the
B string, again tremolo picking, sort of has a mandolin effect.
The picking is tricky. To me it seems you must keep the pick at the exact level of the strings, don't dig in deep it catches the pick, slowing you down, and making the tremolo uneven. Moving from the elbow, not the wrist, seem to help.
One other idea. While not necessarily about tremolo, I have always felt that the guitar with a capo up high, maybe around the the seventh fret and higher, makes an acoustic guitar sound a lot like a mandolin.