You'd be doubling the dose, as the proprietary collars/drops are regulated as a single dose for your animal, would be a waste of money & possibly dangerous/ damaging to your furry friend.
We treat our cat once a month with 'advantage' from our vets. We did try a
cheap imitation from our local supermarket but ended up with fleas so have
reverted back to advantage. Also had trouble with ticks, but thank God he
hasn't had any of them since. They make me cringe ever so slightly!
Advocate is really dribbly (and smells vile) - I had to have lessons from the vet nurse how to put it on. Go with Stronghold - very easy to dib on the back of the cat's neck, and it's effective against one of the worms as well.