I'm so glad to read this thread - I am going through a phase of working out twice a day at the moment, even though I know that it isn't recommended.
I too have a physical job and lifestyle in general, and I still workout 7 days a week every morning.
However, in recent weeks I've thought "Oh, I'll just do a quick workout this afternoon too", and a quick workout has become a full hour in the afternoon, on top of my morning workouts and physical job.
It's really silly, because although I've done my workout this Tuesday morning, already I'm feeling anxious about getting another workout in this afternoon, and it's still only 6.40am here in the UK - how daft is that?!
For instance, yesterday I did an hour in the morning (Firm 500 Calorie Workout DVD), and then in the afternoon did a Jillian Micheals DVD - and although mentally I felt better, almost as if my head was telling me I'd done the right thing - physically I really didn't get alot out of it because my body was just so tired from 7 days a week workouts. If I'm totally honest I know I should not have even considered a second workout atall.
I shall keep remembering the responses of Nancy and Unident today while I'm doing my job and thinking "Must get home for a second workout" - and also keep reminding myself that I shall still be working out tomorrow morning anyway, so how about I forget that second workout today that isn't needed (and indeed may lead to injury) and save my energy in order to really have a good workout tomorrow morning.