Main Motorcycle: '04 Sprint RS
Join Date: Jul 2007
Location: Austin, TX
Other Motorcycle: 2000 Sprint ST
Extra Motorcycle: 2011 Thruxton
I assume that the "Washington" in your location is either the US state or the District of Columbia? If so, you should already have both relays installed, and both lights should always be on whether on high or low beam, with no off switch. The lamps (bulbs) in question are dual-filament H4 types; low beam filaments in both lamps light up for low beam and high beam filaments in both lamps light up for high beam.
The headlight relay mod is for certain non-US markets where only one lamp is lit under certain conditions. If you have a US bike and only have one lamp lit at a time, you most likely have a burnt out filament in that lamp and need to replace it.
As far as HIDs go, the headlight cutout relay (totally different from the relay mod relay) interferes with them to some extent. HIDs need to warm up, and so don't like being turned on, then shut off while the motor cranks, then turned on again. My understanding is that it causes excess wear on the ballasts; it certainly doesn't prevent them being fitted.
Your link to DDM has pop-up menus for several different options, so I'm not sure which you were looking at. The kind you want for a 955 is the dual light setup with H4 hi/lo lamps.