The alarm clocks will beep together again at the Least Common Multiple (LCM) of 20 and 25 minutes.
\[ \text{LCM of 20 and 25} = 100 \, \text{minutes} \]
Thus, the next time they beep together will be 100 minutes after 12:00 PM, which is 1 hour and 40 minutes later, or 12:40 PM.