4th order air velocity Tuning question

I mocked up a 4th order box for 2 subs. When I have separate sealed chambers and shared ported, I get a low port air velocity. Problem happens when I decide to have subs share the sealed section and double the sealed cuft. To compensate. Leaving the sharedported chamber untouched. The velocity almost doubles. 

is this inaccurate?should the air velocity output stay the same when I share the sealed chamber? Is this a glitch in the program?

This can make or break my build as the two subs will be facing the rear of the car and I prefer the open chamber  

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I'm sorry for the late response. I'm a little confused. Can you provide more details? Why did you double the sealed section?