open first input file A
open output file
read input file A (probably use fgets() )
write to output file (using fprintf() or fputs() )
close input file A
open input file B
repeat the above for B
close both files and you are done

You need to study your textbook about the functions fopen(), fgets(), fprintf(), fputs(), and fclose(). You also need to know about character arrays that hold the information in memory that the fgets() function uses.