Working at those sorts of frequencies requires specilist test equipment and knowlage. There is nothing much that you can practally do.
Sorry but those ideas would not work. Two transmitters would have to be synchronised and that needs special circuitry.
Playing with the antenna would workout you need expensive test gear and would void any regulatory approval you have. In nearly all countries you are not permitted to make your own transmitter even for use on a licence free band.
The only thing you could do is to increase the voltage you drive it at. Look at the spec and drive it at the maximum voltage no more.