If you have the parallel kit WITH the 30 amp outlet.. There is no need to get the Companion, the advantage of the companion is you use a cheaper parallel kit.
As to running just one.. Honda says "DO NOT" however, as noted above, many do.
I have never looked close up at the parallel kit,, I will make an assumption that being that the ends of the wires are protected so you can not touch a live wire (kind of like the power plug on the side of my computer) if this is the case I'd make a small weather case to cover the end of the cable with and unplug at least one of them if you only want one unit running. A pill bottle might be employed for this. or (Depending on the wire size) 2 blocks of wood with a gouge the wire lays in and THICK weatherstripping tape on the mating sides, Lay the wire in and clamp it with a spring clamp. 100% protection.