Step 1: Plug in printer A
Step 2: Configure printer A (I suggest calling it something logical such as 'Printer A')
Step 3: Plug in printer B
Step 3: Configure printer B (Coming up with a suitable name could be tricky, but if you've followed my steps so far then 'Printer B' should be available)
Step 4: Print from your app, being sure to select 'Printer A' from the 'Printer:' drop down.
If you're finding step 4 tricky then I suggest revisiting step 2. Did you configure your printer with the correct name?
Of course these steps are assuming you only ever want to print from printer A. If you also want to print from printer B then you'll need to somehow adjust step 4.... I'll let you work it out.