Like i said if there were a safety issue with them they wouldnt sell and fit them check there website Prochem Europe.
Checked their website, and nowhere does it give any info on that, regardless, I'm sure they sell them with their systems, but then one on each side.
Plus, you can't really say, just because they sell them they are safe to use in any way you like it? Every supplier sells tanks, but not every user installs them correctly.
One saddle tank on one side destroys the handling, and just for that reason it's not safe. Imagine doing an emergency evasive manoeuvre , that saddle tank on one side will feel like pulling the handbrake.
As you can see here, you need one on both sides (scroll down) :
http://www.gardinerpolesystems.co.uk/acatalog/Skid_and_Saddle_Tanks.html