I was initially concerned that should the flies die or tire in mid air, then it would inevitably result in a crash (you probably wouldn’t even get your air worthiness certificate). May I suggest an emergency can of spray starch. In the event that things are going pear shaped all you have to do is spray the starch on the flagging or dead flies and hey presto there you have it -- a glider.
Regarding the equal power issue, James watt actually created a method for measuring "Horsefly-power" and called it the "flyno"
So what you need is to is take your vehicle along your local flyno and make sure they are balanced correctly (beware though some flynos are notoriously inaccurate), may I suggest that rather than permanently gluing them on, for the flyno visit you should use blue tack, if there is an unbalanced fly you would have to pull the flys legs off meaning you would need to find another fly of the right power, A good flyno usually has spare flies in stock though.