I will be using 2 EX1s and need to record from a single Mono out from a live stage mixer to both cameras. For my last shoot with 2 cams, I used the Mono out into one camera and the Monitor out from the mixer into the other camera.

One idea I have is to use this mult box Whirlwind | PRESSMITE - Active Press Box | PRESSMITE | B&H Photo

We will buy this mult box so if it will work, then that is good.

You can probably get away with passively splitting the signal, using a splitter box or Y-cable. The line output from the mixer should be able to drive two high-impedance inputs such as the line-inputs on the cameras. But I'd want to test this before I relied on it, just in case. Depending on how the input and output stages of the cameras and mixer are designed, you might see a slight increase in the signal-to-noise ratio.

I concur with Mr. Hughes that you can likely use a passive splitter, even a "dumb" Y-adapter. The press box is massive overkill and a waste of money unless you are really running a press event and expect a dozen reporters to show up without RSVPs.

You didn't identify the mixer. If they are mixing mono on a stereo mixer, then you could just take the left channel out of the mixer and feed one camera, and the right channel and feed the other camera. If they are mixing mono, then the "left" and "right" outputs contain identical signals.

I agree, unless it's an extremely limited rack-type PA mixer, it should have enough outputs to feed two cameras. If it does only have one regular line output, then you could also use a dual passive direct box (or two separate passive direct boxes). Loop the output from the mixer into both direct box channels using appropriate cables, then send mic-level signals to the cameras on regular XLR cables. This would give you the advantage of dropping the ground connection if there are any ground-loop hums, and using hot mic signals into the camera (which some cameras do better with than a moderately hot line signal or a split, weak mic signal).
It all depends on what you can get your hands on either locally or by ordering online, especially in relation to which type of connectors/gender/Y type etc., but unless you need the pressbox for other press conference needs I wouldn't spend for it.