If you can invent a losses combiner, you would hope to get twice the power. Expressed in decibels, that would be 3.01 dB additional gain (rounded to the closest 100th of a dB). Many folks have tried using a common two-way splitter in reverse... but it has at least 3.5 dB insertion loss, thus a net loss of gain.
Now this is interesting >>> 2-way, 4-way, and 8-way gangs are common. Successful gangs of other numbers are very uncommon.
For a 4-way gang, there is a trick for eliminating the combiner, but it only works for balanced line such as 300-ohm twinlead:
The “two in series, two in parallel” connection maintains 300 ohms. If well constructed, there is no loss whatsoever. A variation on this is to connect all four antennas in parallel and use a 1-to-1 balun.
http://www.hdtvprimer.com/ANTENNAS/ganging.html