Sure they can - its called fraud. Anyone can do it.
If the first client has agreed to pay the lawyer for time spent waiting in court, and the second client has agreed to pay the lawyer for time spent working on the document, how is it fraud? Both clients get exactly what they paid for. Neither is overcharged. The lawyer is compensated well.
Efficiency is not fraud.
Or looking at it a little differently, accepting your premise, which client should get the free legal work?
If I have room in my airplane for two passengers and each is willing to pay me $1,000 to fly him to Los Angeles, must I only charge $500 a piece if I take both?
If the clients agree after full disclosure, obviously there is no problem. But if I need to be in court for client A, and can do work for client B during that time without adversely impacting my representation of client A, then I will. But I am not going to bill client A for time spent working on B's matter while sitting in the courthouse, otherwise doing nothing.
The point of my post was (i) to rebut the premise suggested by the poster to whom I responded that lawyers unethically double-bill their clients as a matter of course, and (ii) to inform as to the fact that highly paid lawyers work an absurd amount of hours - evidence of which is the fact that I am still at this hour checking periodically on Freep as I work.
I should have been more precise.
p.s.
"Or looking at it a little differently, accepting your premise, which client should get the free legal work?"
The one for whom you are charging a lower hourly rate! :)
This is called double billing and is both illegal and unethical (“It goes without saying that a lawyer who has undertaken to bill on an hourly basis is never justified in charging a client for hours not actually expended.” ABA Formal Opinion 93-379, at p. 7. You can't ethically expend two hours in one hour You are either working on behalf of one client or the other.) There is actually a notorious case about this...a lawyer who would bill in excess of 24hrs a day, on the theory that he was working on other matters while traveling in a plane cross country.
Now days, most large clients use computer billing systems, and regular audits. It is very easy to get caught doing this, and the consequences are unpleasant.
Heck, some of my client's billing systems will not allow me to enter more than 12hrs a day in time without a special waiver.