Consider:

• less of the two - a smaller amount of both things under consideration: I would prefer to see less of the two in the future.
• lesser of the two - the thing that is smaller in quantity, amount, stature, etc. than the other: I'll take the lesser of the two and you can have the other one.

One way to look at it, then, is that less refers to an amount of something, whereas lesser refers to the thing itself, as a kind of status indicator.

Consider:

• less of the two - a smaller amount of both things under consideration: "I would prefer to see less of the two in the future."
• lesser of the two - the thing that is smaller in quantity, amount, stature, etc. than the other: "I'll take the lesser of the two and you can have the other one."

One way to look at it, then, is that less refers to an amount of something that need not be compared to anything else, whereas lesser refers to the thing itself in comparison to something else (which need not be explicitly named).