Daniel, there's actually no such thing as a cable specifically for a subwoofer connection, although some sellers label one of their cables as such. These are coaxial cables with RCA plugs on each end which can be used for various audio and video connections, including to a sub. The one i've used for about six years with my EP500 is this
. For the two cables for two subs a "Y" adaptor is plugged into the sub out on the receiver and then the two cables are plugged into it. These Ys come in various forms and here's
As far as setting up the two subs, the Audyssey version in your 3808 has one calibration setting for the two(some Audyssey XT32 models can set separate calibrations), so yes, it'd be a good idea to get the levels equalized with your SPL meter as a starting point. Then for the room equalization part of Audyssey, it would equalize the composite bass output from the two subs as one. A good discussion of using Audyssey with two subs is found here
by scrolling down to and clicking on section f.2.