If you have 2 underscores very often and you don't need the underline markup just put the following line at the top of your tiddler. It will disable the underscore wikification.
eg: test tittler \rules except underscore __test have fun! mario -- You received this message because you are subscribed to the Google Groups "TiddlyWiki" group. To unsubscribe from this group and stop receiving emails from it, send an email to tiddlywiki+unsubscr...@googlegroups.com. To post to this group, send email to tiddlywiki@googlegroups.com. Visit this group at http://groups.google.com/group/tiddlywiki. For more options, visit https://groups.google.com/d/optout.