I'd be more concerned about the kicker the top pair comes with than the odds of it being flopped.
http://www.homepokergames.com/odds.php
in the Hand Matchups section places Top pair(bad kicker) vs Top pair (good Kicker) at 12.7% vs 87.3% underdog.
I'm not so sure this is even a calculable scenero without specific values known upfront. i.e if you had A-7o either could flop top pair but one would have a great kicker while the other has a lousy kicker. The odds of flopping either the A or the 7 could be calculated but how would you calculate 2 random unpaired pocket cards? Not sure it can be done.