How flexible are number-of-pages submission guidelines for conferences?
None of the CS conferences I have ever submitted to had ever been lenient on page limits. There are often automatic checkers that will not allow you to upload a paper that exceeds the page limits. Conference guidelines are usually very clear and unambiguous, go and read them. Some conferences allow an appendix or a link to additional material that reviewers can read at their discretion. 

Regarding splitting the papers - if you can do so in a clean way, and both parts make sense (tell a compelling narrative, have meaningful results) then sure. Otherwise, expect two rejections.