In short, this modification is not possible without some extensive theme modifications. To do so, I recommend reaching out to a NationBuilder-certified Architect. If you'd like to change the truncate limit for event pages, you can do so by editing the "_event.html" file in your custom theme. If you do not yet have a custom theme, you can create one using these instructions: http://nationbuilder.com/theme_documentation

Instructions for web developers comfortable editing Liquid/HTML/CSS code: 

If you'd like to have the event page functionality act similar to the blog post pages in that people could decide for themselves where the truncation happens, you could use the following walkthrough in the "_event.html" file: http://nationbuilder.com/how_to_embed_widgets_code_iframes_between_the_content_text_or_body_text_of_a_page_or_email_blast

In other words, you could make it so that when creating an event via the control panel, adding the term "SPLIT" in the middle of your event description would let the site know which section of that description was "before the flip" and which section should be "after the flip".

You could then make a corresponding change to the "user_event_pages_new.html" file in your custom theme so that two text boxes show up, one labeled "before the flip" and the other labeled "after the flip". You would need to add some Javascript so that the two text boxes concatenated together with the word "SPLIT" between them is submitted as the event description text.