Though it's hard to know how much of Jackman's trouble with Valjean had to do with singing the score live on a film shoot for weeks on end. I felt like he got noticeably worse the later in the film we got, and my memory is they shot more or less in order.
But I don't think any actor is good enough to pull off "You're Nothing without Me" singing a duet with themselves... nor would that necessarily be appealing to watch.
