I was a bit skeptical at first. You have obviously looked at the bidding history on a number of lots. If these two bidders have bid on essentially the same lots consistently in the manner illustrated, I would think shenanigans. If neither of these two bidders are associated with the lot owner(s), or the auction venue, I wonder what their game is. Which makes me think they are associated with the owner(s) or the auction venue. Now, if they have bid on, say, 1000 lots and overlap each other on 'only' 500, it could easily be a couple of aggressive bidders that coincidentally have similar interests. If they are bidding on 525 lots, and overlap on 500, then I think there's something rotten in Denmark.
I understand that online bidding usually results in few bids until the last day/hour/minute when the price skyrockets. I am not alone, though, in that I often put in a healthy bid (not actually my high bid, but close) early in the bidding so that the lot shows up on my Bidding List. I see it as a reminder to come back 10 minutes before it sells to put in my final bid. I could Watch them instead, but I don't. My point is, although it is a bit unusual, there ARE people that bid early.
I don't know if there are shenanigans, but I would certainly be uncomfortable with the situation. Out of principle, I would walk away.
You COULD inform the auction venue of what you have found. If these bidders are fraudulent AND not related to the auction venue, you may get some traction. If they ARE somehow associated with the auction venue, you will get lied to if they respond to your notification. They may not even respond. You may want to keep an eye on how these lots sell. -To one of these bidders? To others, with these as the underbidders? Are their bids cancelled at the last minute?? Also, do these lots appear in a not-too-distant-future auction?