Chance of Durant choosing the Celtics is 1 in 30 which is 3.333%
Chance of Horford choosing the Celtics is 1 in 30 which is 3.333%
Chance of Durant and Horford both choosing the Celtics is 1 in 900 which is 0.111%
Those are your real and true mathematical probabilities of the events occurring.
Doesn't actually inspire hope, does it?
BTW, just to put this in perspective it is 5 times more likely that a 14h worse team in the league wins the draft lottery than it is the Celtics landing both Durant and Horford.
prove it. i mean it. show me your math.
mathematical probablities are the MEASURES of the probablity of something occurring, and is most commonly measured with a number between 0 and 1. from this, you draw inferences based upon the expected frequency of events. the number and actions of variables are known.
the key is the word MEASURES, not guesses, hopes, dreams, thoughts, or that ilk. you can measure the probability of a die coming up as 1 since you a finite set of outcomes (6) all of which are equally probable. or, as with a coin flip, the probablity is MEASURED as 50% for both heads and tails based upon total number of possible variations or outcomes.
coins, dice, etc. dont have bias (at least not legally.
) therefore one can MEASURE mathematically the probable outcome of events. coins only have TWO possible outcomes and are unbiased as to which side will appear. hence the 50%.
i dont see how the factors that go into your calculations are unbiased. quite the opposite, they are by definition suppositions and assumptions by a large number of people.
if you want to say "x is more probable that y because i think this way" i have no problem with it. but let's not dress such a thing up as some sort of mathematical formula folks.
whew...my morning curmudgeonly-fit is all over now. 
Actually I think you have things backwards. In this instance, there are 30 teams from which both Horford and Durant can sign with. Therefore there is a one in 30 chance of Durant and Horford choosing any one team and the probabilities of them choosing the same team is one in 900. That's the math. That is indesputible.
But because there are human choices involved with preferences and biases, mathematics is about the worst way to determine the handicapped odds of Durant and Horford signing with the same team. It is the introduction of human bias that creates a different playing field for determining the "odds" that they both sign here. Such biases are:
Preference to maximize earnings
Preference of certain living conditions
Preference of certain working conditions
Those things, when taken into consideration narrow the choices and can come up with a different set of "odds" for determining the likelihood of Durant and. Horford choosing Boston, but it does not change the mathematical probability of them both choosing Boston. That answer is still 0.0011111