Do you mean importing all the tables, relationships, layouts, scripts and value lists from one file into another?
That can be done, but if you have a lot of the above elements to import, this can be a major job with limited benefits. There's a utility out there called FMMigrator that claims to simplify this operation.
If attempt this with just filemaker...
Get Filemaker Advanced if you can possibly scrape up the extra $$. The database design report becomes very helpful in checking to see if you've broken any part of your system while merging your two files.
Tables and scripts may be imported
Layout elements can be copied and pasted from one file to the other after the layout parts have been properly defined and sized to match the original.
Relationships and Many value lists have to be redefined from scratch to match the original.
The values in custom value lists can be copied and pasted.
Imported tables may encapsulate come field calculations in comment brackets until you add the missing items referenced in them and then edit the expression to remove the /* comment brackets. */
Scripts often refer to layouts and layout objects. Layouts refer to scripts and value lists. Since you can't import both at the same time, the resulting layouts and/or scripts may have broken references to items that do not yet exist. You often have to create the layout and value lists, import the scripts. Delete the layout objects and copy and paste them again from the original file to avoid the broken references.