I am sorry but you are wrong, the strike rate is the Combined strike rate of the second and third favs.
Unless you mean something else, as far as I know, the strikerate is defined as how often you hit the bulls eye or snare the winner.
Betting on more than one horse is almost like using a machine gun as opposed to a rifle, bet on all runners and you will have a 100% strikerate (and an empty bank).

Now if you talk of the return on investment you are correct, you outlay double, so your return is accordingly the (win div - $2.00) instead of (win div - $1.00). That is for each bet, meaning that you bet two horses!!!