go to arial unicode, and its there. you have to look in symbol in that font or it wont work. i just found it. here it is---- ⇌ its a bit small.
Your first post is a resurrection of an over 3 year old thread. Nice . I wonder how many pages you had to go back to find it. Anyway, on the topic, before I got to uni I used to do a google search for "reversible reactions" and copy and paste the symbol from a random website. I could resize the symbol so that it was in line with the rest of the text.