In principle you can use a "standard" directional coupler, if you just want to mix two different signals, but depends from your application (f.i have you got some specific phase impairement requirement ?) and the maximum attenuation you can afford.
Take into account that the signal on the direct line will be attenuate by just some tenth of dB while that sent to the coupled line will be attenuated by the coupling factor. To equalize the attenuation you have to insert a fixed attenuator from the generator toward the direct line.