You could have as many .5's on a plane as the game permits, but no more than 2 .5's on the same route.
Wait a minute.. Every plane I have doesnt allow me to have more than 2x 0.5 routes, e.g. Plane A has 10 routes, 8 of which are 1 freq. and 2 are 0.5 freq; it's just the game not allowing me to create another 0.5 route when I already have 2 or maybe 3 in the current plane, can you check it out please ? ID 3573
Yepp, its the game.
Normally you can create 2x .5 routes. May those be 0.5 or 1.5 or 2.5 etc. If you go from your first hub to your second hub and to your third hub (or any destination) you may have more.
Examples:
A, B and C are your hubs and D is a simple destination
A -> B -> C -> A ; 3x .5 routes allowed per plane
A -> B -> C -> D ; 3x .5 routes allowed per plane
If your bases are close together you can even create a roundtrip with .5 routes.
^^If this is wrong I misunderstood that rule 
you're right, though in fact you can take it out a step farther.
if A, B, C are hubs and d, e, f, g are other destinations.
d->A->e->B->f->C->g is valid [all .5's] in game.
if your plane is flying into and out of only one hub, yes the cap per plane will be 2 .5's. this type of cap is enforced in the code.
regardless of the above, the cap per route [player wide] is 2 .5s, this is enforced outside of the code.
