将大型Excel文件复制到500行较小的文件中

I have the vba code below in Excel that almost works. I'm having an issue where the "firstrow" isn't increasing. The first file will contain rows 1-501, the second file will contain 1-1001 (I want it to contain row 1 & 502-1001). I'm sure it's a small issue but I've stared at it for a while and can't seem to figure out whats wrong. Some help would be really awesome!!