Some divers do recreational dives with double tanks. If you have a total BC failure at the start of a dive with full tanks then you should be able to ditch enough weight to get neutral on the surface. This doesn't necessarily mean carrying extra weight with doubles, but relative to using a single tank it usually means shifting more of your total weight to be ditchable versus non-ditchable. And it becomes even more critical to have reliable gear to avoid accidentally dropping weights.Why is this? Where does this rule of thumb come from? Why is it safe for a diver with two tanks to have double the ditchable weight that a single tank diver does? I am really curious, how this guideline has been developed and exactly what logic is a being applied, when we are talking about a recreational diver?
Anyway I don't want to take this discussion totally off topic for the Basic Scuba forum. Suffice it to say that assembling a balanced rig becomes more complex with multiple tanks and if you want to get into the details it would be better to ask on a different forum. Search the archives for "balanced rig" and you'll find a lot of prior discussions that should get you clear on the basics.