I know that a lot of people *want* to get both the Bladder of Steel and the Platinum Artist at the same time... and that's admirable. The ESL2 takes so friggin' long to do... why would you want to do it TWICE?
Well the answer is simple... when you hit the brick wall that is Visions (explained well above), you may end up losing both achievements at one shot, 5 hours into your session. And that would suck. (Believe me, I should know... a few days after getting the ESL2 on vocals, I tried the Bladder, and got interrupted near the end because of RL, on two separate occasions! I'm looking to try it again soon.)
If you think Visions isn't so terrible, then this warning doesn't apply to you. However, if you're like me, you'll take the more conservative approach and do the two separately. Platinum Artist is quite a bit easier on vocals than any other instrument. The problem with vocals is that by singing for over 6 hours straight for the Bladder, you literally will hurt your voice, and could fail toward the end just because your throat gives out. That would also suck.
Instead, my suggestion would be to do them in two sessions, as painful as that may be to hear. Assuming you're not a Rock God (in which case you probably aren't reading this guide anyway :-), you can get Platinum on vocals (or bass, with caution that you may fail Visions a few times along the way). That way you can take breaks. (For my ESL2, I think I actually did it over 3 days, stopping whenever my throat was getting sore.) Then in a different session, you can get the Bladder playing a different instrument on medium, like bass.
Don't strain yourself, though. The two achievements are separate. The Bladder is cruel enough as it is. If they had a Platinum Bladder, now that would be quite the challenge. In the meantime, take the two separately.
(Those of you leaving comments about how you did them both at the same time, seriously, my hat's off to you. Congratulations!!)