There are true twins and directional twins.
With a true twin, your distance from the nose to the first insert set will be the same as the distance from the tail to the last insert set. The stance is not setback from the sidecut.
With a directional twin, again, the stance is not setback from the sidecut, however, the distance from the nose to the first insert set may be bit longer than the distance from the last insert set to the tail.
With a freeride board, the stance is set back from the sidecut, and the nose is usually a touch longer.
Unless a company puts their inserts in an odd way, this should all be true I think.