What errors though?
Perhaps it becomes full of errors because you have 2 instances of the component names and net names?
First re annotate the 2nd schematic so that the numbers start "after" the first schematics ones.
I am not sure about net names, if the system nets will past in as new names, any named nets may connect so need checking and possibly renaming first.
Then copy and paste.
If it goes in but with errors, can you list, check and fix the errors leaving you with what you are wanting?