Too simplified.
Either model is not rated for a max 30 Amp load. That's the reason you can't have just the one with twist lock (2000i Companion model). Either one is only rated 2kVA.
I've just been to Camping World and bought the Standard 2000i. I asked for the Companion with the twist lock. . . and Tech support told me I can't have the one with 30 Amp. and expect to hook it up as a single unit and draw 30 Amp energy. I have to have the Standard 2000i if I intend to run only one unit.
The Companion has to have another 2000i (well, as Honda says), it has to have a companion.
You can verify with tech support and find out for yourself. Let me know what you come up with and I will return the unit I just bought.
The reason I wanted a twist lock plug is, I have my old system with 30 Amp. twist locks. . . obviously that didn't work out for me.