I would keep the liine length between the power combiner and the DUT very short. Then I would put at least a 3 dB pad on the two inputs (more is better). You will have some mismatches, but they will be not too big of a problem.
At the DUT output, use a 75 ohm pad once again to keep VSWR standing waves to a minimum.
Of course, you could also use either all 75 ohm components and test gear, or matching 50 to 75 ohm coaxial pads and just take into accout the published loss.