I cannot see anywhere in prescribing information where doubling the daily dose was recommended, sometimes doctors just find out variances that work within their own practice. I did see reference to 4 Gm daily by enema in studies to induce remission for left sided colitis. Overall, mesalamine is quite safe and there are few side effects even at high doses. The main problem with twice a day dosing of suppositories is rentention of the medication when you won't be laying down, since ideally you want to retain it for a minimum of about 3 hours. If this is a problem, you may want to discuss going back to once nightly suppositories and adding oral mesalamine. This strategy was found superior to oral or topical separately to treat left sided colitis. The main thing with higher doses and ileal tissue exposure is that there will be more systemic absorption, so periodic liver and kidney screening is advised.
This is a good article on the topic. You have to register with Medscape to read it, but it is free.
http://www.medscape.com/viewarticle/578265
Hope this helps.
Jan