Combining audio outputs once amplified
I would use option 2.  That way you don't have to variable-attenuate power.  You can use a single pot to blend between the outputs of the two radios.  The output of this will be much higher impedance than a speaker requires, so you feed that into a power amp to drive a speaker.  You may have to attenuate the output of the pot a bit to keep the power amp from being overdriven.