Hey Hidde, its 4 hexes
between BG side 1 vs BG side 2 NOT counting either hex either unit is in. Interestingly , a unit that started with 4 hexes betweem itself and an enemy can double move AWAY from that unit ( as long as it keeps its final destination 4 hexes away(between) an enemy bg , however a unit that starts with 3 hexes in between an enemy BG is denied double move altogther (used the scenario editor to play around ) Kinda confusing!
I have seen the phenom that Morbio so easily put togther with his camera

before occasionally. So i think in the pic its just a graphic issue, Morbios cavalry should not be allowed in the last highlighted hex, but should be allowed in the second last with the arrow in it.