Answer:

Only one file can be associated with one file number.

One File per Number

It is perfectly fine to open more than one file. Here is the corrected version of the program:

CLS                                  ' Clear screen
OPEN "MYFILE.TXT" FOR OUTPUT AS #1   ' Open the file for output
OPEN "URFILE.TXT" FOR OUTPUT AS #9   ' Open the file for output
PRINT #1, "Hello World"              ' Send characters to the first file
PRINT #9, "Hello Mars"               ' Send characters to the second file
END                                  ' End the program

The file numbers are arbitrary (you don't have to number files #1, #2, #3 ... in sequence).

This program will write out the first string to the first file and the second string to the second file. You could continue writing strings to the two files in any order.

QUESTION 11:

Here (again) is a description of the OPEN statement:

OPEN file$ FOR mode AS filenumber

file$ is the name of the file. 
mode is INPUT or OUTPUT.
filenumber is a number 1 through 255. Use a separate number for each file.

Could the file name be contained in a string variable?