That brings up an interesting point: can two lead characters move to another show and it not be considered a spin off if it was a deliberate spin-off? After all, if said characters can exist in the same universe as the characters on anotehr show, this implies it is the same universe, which is what a spin-off really is. Think "Angel": it occurs in the same universe as "Buffy the Vamprie Slayer" but takes place elsewhere and brought, over time at least two leads and two more secondary leads into the mix; same unvierse but doing other things.