Same selling agent or different? Any idea how many people are interested?
It's a tricky one as you run the risk of getting into a situation where two parties bidding on both only serve the drive up the prices on each other. So definitely avoid being the lead bidder on more than one of them at a time.
What Brendan said 100%. Plus estate agents know their local market and may even talk to each other. I'd also say be very upfront with the agents. If you're a serial viewer and never follow up with a bid, or never explain your situation, they'll pigeon hole you as a tyre kicker and when you do go to make a bid, they'll take you less seriously.