port and sub firing direction

Best choice is to port to the same side as the driver, whether front-firing or down-firing.  Alternative is to port in another direction, and this is dictated by the amount of space available for the port structure.  All the sound will come out into the room, so it's not that big of a deal.

If you want to convert one port to two, and keep the same tuning frequency (of course), each (equal in diameter) vent's length will be slightly more than twice the length of the original single port.  This is due to the fact that doubling the number of ports greatly increases the port opening area.  If the dual ports are made to be smaller in diameter than the original port size, you will need to calculate port opening area in square inches.  Port calculators come in handy for this, as well as box building programs such as WinISD Pro.

http://www.subwoofertools.com/forum/portlength.htm

http://www.subwoofer-builder.com/freesoft.htm

http://www.mhsoft.nl/spk_calc.asp#top

http://www./help2/Articles.cfm?id=31

...and to the left side of this page under "port length calculator"...

I've always taken the sub and fired the port into the corner of the room. It seems to roll up the wall and back over the room, in my opinion... As far as ports, I know that the length of the port is what determines the frequency, I believe If you take the diameter of the port and divide that by two, but keep the same length, that will give you what your looking for. If this was me, unless I was going to put a divider between the subs, I'd just run one single port.

Home audio and car audio are somewhat the same, but yet pretty different in the same aspect. What type of sound are you going for, because I've seen some excellent sounding surround sounds with a single 8" sub....