RF Combiner Hardware
It's called a power combiner.  There are several different types, and the idea is to make sure that all of the connections are properly matched so that you don't get any issues with reflections under normal operation.  For example, if you use a BNC tee, then both sources will see a 25 ohm load and a very significant portion of the signal will bounce off of the split and come back to the sources.  A device called a wilkinson coupler uses two 1/4 wavelength pieces of 70.7 ohm coax and a 100 ohm resistor to carefully cancel these reflections over a narrow bandwidth.  There are other types of couplers and power divider/combiners, but the wilkinson is pretty simple and easy to make.