You certainly can, in theory, use multiple transmitters together to produce a signal of higher total power. However, there is a catch: the signals must be in phase with each other. Otherwise they will cancel each other out instead of adding, half the time.
To have the same phase, they would have to share the same RF oscillator (or be coupled to a master oscillator, perhaps), and the feed line lengths between each transmitter and the combiner must be equal. A more practical way to look at it is that you can build one transmitter which has many amplifiers that work together to produce a high power signal. This can even be a particularly useful technique if your amplifier components cannot tolerate the voltage, or heat, that would result from the total RF power you want to transmit — each individual amplifier works with a lower voltage and can have its own heat sink.
A related technique is beamforming, where you use multiple amplifiers and multiple antennas. The waves from each antenna interfere with each other, constructively or destructively; by controlling the amplitude and phase of the signals transmitted via each antenna, the antenna array can be “steered” to transmit signals in a desired direction, without need for any mechanical rotation.