Just dial the country code for the UK (011) then their phone number.
When they answer, tell them it's an international call from the US, and hopefully they don't put you on hold, or if they do, you'll get top priority. That's what I did when I phoned my order to NWS in the UK.
As for the dual can setup, it's not gonna get you any more power and there's the the weight penalty of the extra can & plumbing, so it'll be strictly a "for looks" thing. I haven't heard a B12 with duals, but I can't imagine it would sound retarded... Hayabusas and other inline fours with duals sure don't. The sound should be about the same, just coming from two sources instead of one.