Difference between using one circulat port, and two

Hi I am building boxes for my tannoy gold monitor LSU/12/8 speakers. 

The internal dimensions are 150L and I've read to tune that size box with these speakers with 2 100mm x 200mm ports.... which is what the calculator is giving me for 2 ports tuned to 35hz.

however that brings the port resonance down to 509hz, with one much shorter port the resonance is around 1000hz...

What would the difference between the one port and two port set up be if its tuned to the same 35HZ value?

Thank you so much!

Hi. It's because when the port becomes much more lengthier than its diameter, It becomes the transmission line. The same we can say about the box when any side of the box becomes much more lengthier than other, The box becomes something like the transmission line but with closed end.