Lots of lifters train 2x a day. Look at Bulgarian olympic lifters who train multiple times a day, handle enormous volume, and who's exercises tend to hit the entire body (full cleans in the AM involve some major front squating and a back squatting PM session hits it again for example). They have been very successful in competition (at least when they don't get DQed for drug tests). I'd venture their elite lifters probably use quite a bit less drugs than most experienced users on this site - but also keep in mind they have spent years building up major training tolerances (natural lifters can also routinely accomodate mutliple sessions per day). Essentially, don't dramatically up the total volume just split it up logically and it should be fine - you might even find it preferable as you are able to restore between sessions and perform better on exercises where you would typically be a bit spent by the time you got to them in a single session.