Subwoofer Cable Question

[allstar780]

So I'm at work right now and I have a customer who wants to run 10-12 meters of sub cable through the wall and has chosen Monster for the sub cable because of the shielding, as it is running up through the wall, along the supports in the roof, and back down the wall (he's an electrician and he's doing his own install) and basically Monster only makes up to an 8m sub cable and the standard sub cables he's used before (monoprice and the like) don't have enough shielding for him... so basically I'm wondering if he were to couple an 8m and a 2 or 4m together in the wall, is there any kind of coupler that would be CL2 rated, as well as secure enough to guarantee it will stay together in the wall?