When I was a lone "engineer", I once had an Exchange issue that I couldn't solve - partly because I didn't then know the answer and partly because I was too stretched for resources to devote the time needed to finding the answer.
Anyway, my employers agreed that in the short term they'd call a consultant in to help me fix the exchange issue and they'd hire me an assistant.
On the day the consultant turned up (he was one we'd used before and I knew quite well) I started explaining the problem to him, and yes, the sheer act of explaining the issue to someone who understood it and needed the technical details caused the probable fault and a solution to pop into my mind. The consultant started laughing when I told him this and claimed he could actually see the lightbulb start glowing over my head even before I told him this.
So it works with more than one person, and I guess with the rubber duck if you can bring yourself to explain the problem in proper detail to it.
The consultant was still useful; given how little time I had, he spent two days proving my latest theory and then implementing and testing my solution. It was still a two person job anyway and that's something that rubber duck problem solving doesn't help with.