I looks like Simon will also use the Combat Cross
nah, its confirmed that simon has the "vampire killer" while Trevor has the combat cross.
The vampire killer in this game is most likely a magical whip made of alchemy like in the old canon and thus it can probably change form.
My guess is it is a upgrade that you find later in the game that lets you powerup your leather whip to the chain one.
It shouldn't be called the combat cross because its uses a chain.
Its still the vampire killer.