I just got a nom from my senator (CT) and am planning on interviewing for another one from my Congressman in late November. First, would they already know that I had received a Senator's nom?
If our current Sen/MoC follow the same method they did two years ago, then yes they will know. My Mid had sent packages to all three but after getting a nom from Sen #1 was called by Sen #2 and told that, to maximize the number of noms, they only give one nom per person per SA. The rep did ask why she wasn't interested in intereviewing for another SA; she said that her LOA + the nom = appointment, so no reason to take a time slot/nom from another CT applicant. A few days later she got a call from our MoC offering congratulations. So yes, the share their lists.
And second, would knowing that I already have a nom put me lower on the Congressman's list of people? Am I at a disadvantage of having a nom befire I interview with the Congressman?
Again, unless things have changed (and my recollection is accurate), you're unlikely to get more than one nom from the two Sen. I would think that a nom from the Congressman would depend on how many worthy applicants they have. If less than 10 then you're likely to be on the list but not the other way around.