Don't snort it, Concerta has a time-release mechanism and will probably work less well that way than taking it orally.
Are you asking this question as applied to writing this essay specifically, or about dosing and tolerance in general? If you're just talking about the essay, I take it you're planning on pulling an all-nighter for this? I'm personally not a fan of methylphenidate, but for the stims I do use (mainly (ar)modafinil), if I need to work through the night I'll generally redose in the mid evening with the same dose as I take in the morning. This isn't something to do on a regular basis though, sleep deprivation definitely screws you up no matter how many stimulants you take, affecting your mood, judgement, anxiety levels, executive function, etc... I recall a few studies showed that people who were sleep deprived for only 24 hours actually performed as badly or worse on several criteria as those who were drunk; in other words, definitely NOT something to do before an exam.
If your problem is general tolerance, speak to your doctor about it. Again, I never got on with methlyphenidate, but I suspect he'll up you to 36mg (18mg is fairly low anyway). Side effects do tend to become more noticeable with higher doses, even with a tolerance, so be prepared for that.
