...whats the worse that could happen? does anyone know?

I really don't think you can do this...

It's a no-no really. If they both get accepted and subsequently published, you are bound to be found out. You can always approach another journal if it is rejected by the first. If I were you, I would withdraw one asap.

I would agree this is a no no. If you get caught I would think there would be serious consequences and your name will be torn to shreds in academic circles.

Dont do it!!!

Oh no, i havent done it or anything!! Its just that Im always being told by my supervisors etc that its a big no-no, but no-one has actually explained what would happen if anyone did it (obviously withdrawing one before it got to the publishing stage). So i guess i just wondered if anyone had?

I was at a workshop on publishing yesterday and as the others have said, this is a big no no. The reason is that you will piss people off no end because of the time and effort they have spent liaising with you and sending it out to reviewers etc, and this could have an adverse affect on your future career. I was told that when submitting an article aim for the most prestigious journal you can then if it is rejected send it to the next one on your list and so on. But only send it to one at a time. If however you are kept waiting for a long time, you can then contact the editor and say you want to withdraw your article. This could have one of two results - if they do want your article then they will probably try to reassure you so you don't withdraw, otherwise you can withdraw and then send it to another journal.

At the point at which you hit 'submit' you will probably have assented to some kind of copyright agreement. Two different journals cannot both hold the copyright of your article, so that is a practical reason why it's not permissible.

But, as others have said, the process of reviewing a manuscript for publication is not trivial, and if anyone involved discovered that their efforts could be rendered pointless because another journal published your work first, you wouldn't end up with many friends in publishing/in your field.

I agree with the strategy of aim for the highest feasible journal and work your way down. But do be honest with yourself about what is feasible - otherwise you'll have to go through multiple rejections. Always good to get guidance from your elders on this one.