Two mid woofers box design

Hi all of you.
I bought 4 Scan Speak 15W/4524T02 at 4 Ohm. I am building a two bookshelf boxes with 2 mid woofers and one 8 Ohm tweeter. Should I just double the box volume designated for one speaker and should I just double the port area and keep the same length?
Thank you in advance.
 
I added the drivers into BassBox Pro to give you an idea of box volume and vented vs closed.
 

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Thanks bradleypnw. I really appreciate your effort. I just discovered that you didn't send me an example but you have inputted the correct data from my drivers. Only thing I saw that the calculation is for parallel wiring at 2 Ohm, but I am wiring them in series for impedance of 8 Ohm. I would like to use a single pipe 39 mm diameter. I will be so thankful if you can run it one more time all the same just please change the wiring to series.