Let's do an experiment
We have two Fas files, tmp1.fas and tmp2.fas
Load tmp1.fas in Acad and it will display "this is tmp1.fas"
load tmp2.fas and display "this is tmp2.fas"
Run cmd.exe, open a DOS window, and enter copy /b tmp1.fas + tmp2.fas tmp3.fas
We get a file tmp3.fas
What will be displayed when tmp3.fas is loaded in Acad?