Mikuni Dual 38's Uneven Pop-off?

When checking the pop-off for my Mikuni dual 38's the rear carb with the fuel pump is not popping, only the front one. I am testing them both at once with the fuel line connected between them and the goal being that they pop at the same time/pressure.
Is it normal for only the front carb's needle to open when testing this way?
Is this the way I should be testing them, or should I test them separately and as long as they have the same pop-off pressure am I good?
 
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