Port Question

Look again at that slot length.  The total length is 24" (notice that if 24" were the length of the first port lenth before the turn it wouldn't fit into the 15.5 external depth given for the box...)

That might change your way of thinking about your needs for length and turns of the port, but your first question about changing a port to two ports still needs to be addressed.  14" X 1.75" represents an area of opening that is meant to be large enough to prevent port noise.  If you divide that into two half-size openings.....hmm

Theoretically it should be okay, because each port (same length) would be dealing with half the air volume.  A drawback to doing that is the increase in port structure that you will have to add to the overall enclosure volume because the total sum of port walls will increase.  I wouldn't make any changes like this unless I looked at port noise using a modelling program or other port noise calculators.  Here is a calculator that you can use to experiment with in seeing what changes are needed if you add a port. 

As a substitute remedy, if the height dimension of 14" doesn't fit into your plans, you can reduce it and compensate with a wider slot dimension so that the port opening area remains the same square inches.  The length would remain the same and you would still have only one port.

And as far as "going wrong with the JL recommendation"....of all manufacturers I would guess this one to be on the money.  They even state that this recommended enclosure was tested in a real-life buildup and wasn't totally reliant on computer simulation.

http://www.jlaudio.com/subwoofers/pdfs/12W7_MAN.pdf