As has been discussed in other threads, when faced with the choice of either placing two uncorrelated bets or of parlaying the two, a Kelly bettor unconcerned with wagering limits or any other exogenous factors should generally choose to take the two straight bets.
Clearly, however, this isn't always true. For example if we know that two bets are each guaranteed to win (perhaps you've borrowed your uncle's time machine) than optimal strategy would obviously be to risk 100% of bankroll on the parlay, completely ignoring the singles.
Looking at a (slightly) more realistic example, let's say you're considering two uncorrelated bets each at -500, each with the same edge. If you could only bet the two straight bets OR the corresponding parlay (but not both, in other words you may NOT bet the singles AND the parlay) how high would the edge on each bet need to be for the parlay to represent a superior choice for a full-Kelly bettor?
(Kind of a pedantic question question, but someone brought it up with me in an e-mail. No prize this time, sorry.)