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I'd think about a lower offer if it was on with two (the place I had an offer accepted on was). It also splits the interest of the estate agent, the priority for them becomes the sale and getting it off the market rather than the price (so the other agent can't sell it) so you can put some pressure on to get them to convince the vendor to accept a lower price (a slightly lower commission is better than none for the agent).
Realistically though I think it's only worth it if it's two significantly different agents. A couple of high street agents in the same area are going to be working the same customer base. An online only one with significant reach or an agent out of the area (but in a similar area) may work to get people who weren't already looking interested.
As a buyer when I see somewhere on with multiple agents, I always assume there’s something wrong with it