You can't just run the outputs of the two pedals together & expect everything to sound right & be happy. That being said, depending on the exact design of the output circuitry in each pedal, you could get lucky.
What you should be able to do is run the output of the BOSS pedal directly into both inputs of your two distortion pedals. Use the A/B box to select one output or the other of the two distortion pedals and pass that down the line.
As long as the output impedance of the boss pedal is low and the input impedance of the two distortion pedals is high, you should have absolutely no problems and no difference in tone.