I've also done dual tube ports firing out the rear like this...

 

Standing waves are created when equal forces impact eachother. placing the subwoofer at identical distances from each side of the enclosure is the most certain way to create them.

Most ports are either at 0 or 90 degree angles. 90 degrees can be absorbed with such low frequencies especially when there is no cabin gain. 180 degree ports essentially recreate the phase disparagement that the port is supposed to eliminate (the port length is meant to delay the rear wave of sound from reaching open air the exact amount of time it takes the forward wave to become in phase with it which is why ported boxes lose response as you gain distance form their tuning frequency).

The time it takes the front(cone out) wave to reflect from the trunk into the the cabin will be similar to the time it takes the rear(cone in) wave to exit the port(already facing the cabin). its not exactly the same but it will definitely mess up phase at or very near the tuning frequency.

The outcome is dull bass that cannot be corrected. Home theater systems dont have to deal with cabin gain and since most rooms are made of perpendicular walls, the front and rear wave will reach the listener at close to the same time regardless of the angle of projection. If you just want rumbling you will never know theres a problem though.

 

Regardless of the physics behind it, I've done boxes similar and they've sounded fine. Like I said, I did the dual tube port design above in my convertible and it sounded great, did 140dbs. all day.