Hi,
I'm afraid Euro has his information almost totally wrong. It is a problem and you should seriously consider emptying the tank...
Diesel is in reality heavier (denser) than petrol - so it's the petrol that would float, but that doesn't matter because unlike oil and water, the two fuels are totally miscible in all mixtures.
That means that petrol and diesel won't form layers - they totally and almost instantly mix into one fluid.
In olden days petrol was added to diesel to stop it waxing in very cold weather - and that was only needed if the garage or you were caught out by a cold snap and hadn't filled up with "winter" diesel.
About a 5% blend of petrol in diesel would stop it waxing (blocking the fuel lines) in temperatures down to -15