Ah, but liquid metal would travel differently, and this isn't just any old well we're talking about.
From what I remember, Ruin controls the mist spirits. The mist spirit would have tried to attack anyone that tried to get to Preservation first (via the liquid-metal lake) because they want everyone to go to the well with Ruin in it instead. Release Ruin, and thus let him have his fun.
Why was it that Vin could release Ruin? No idea. Maybe only certain people can release Ruin, but anyone could release Preservation. Otherwise, why would the mist spirit attack Elend and Fedik? They'd only have to worry about Vin if the "opening" possibility was equal between the two prisons.
And why are both "Gods" trapped in the first place? They must have been put there by someone. Presumedly, by trapping Ruin first, and in the process having Preservation follow along because of balance or some such thing.
And liquid metal is mercury. For the most part, Brandon has kept to "real" metals. So, unless he's made up something else entirely for this one, that's probably what the liquid metal is. It could have something to do with traps and/or prisons of godly constituents. And who's to say that the stuff in the Well, trapping Ruin, was water? Maybe it was mercury's opposite-twin. A clear metal? Now,that'd be cool.
To tell you the truth though, the whole thing about the liquid metal lake went over my head when I read WoA. Just wasn't obvious enough I guess. By the end of WoA, everything was moving fast enough that even the connection between what Vin and Elend find and what Fedik found wasn't clear for me.