How to divide one EndNote library to two absolute libraries

There are too many files in one EndNote library I am using. If I want to divide it to two libraries including the attached files, how shall I do it?

Why do you think there are too many? My library has over 5000 records.  What version of Endnote are you using?  Why not use groups and keywords to internally separate your library into topics.  Splitting a library creates some problems, you have to remember which library you put something in, or you end up duplicating records that cross topic boundaries.  

If you still want to do this, are there some critera you want them split between?  If just some sort of chronology, make a copy and sort on the year.  On one copy delete the first half, and on the second, delete the second half.  The two libraries will not at least have the same record numbers for each of the records.  Since X3 you can also export parts of libraries to an .enlx compressed library, and if you open that sub-library, it too will retain the record numbers of the original.  

You are right. It is not necessary to split the library.Thank you very much! Btw, my EN version is X2, why I can not find the function before.