The odds of a character being BOTH is something like... one in 2000 or one in 5000. That is ridiculously probable, compared to seven consecutive natural 100s.
It's like the probability of getting in a car accident some time this week, compared to the probability of winning the lottery AND getting struck by a bolt of lightning, IN THE SAME WEEK.
...wow.
So, the odds of actually getting one is one in 10^14. That is, one in one hundred trillion
In the Exile population, assuming limited to no further population growth (population under a million), this means we can expect a Legendary Super Saiyan every... hundred million generations or more, that is to say the planet Garenhuld is fairly likely to die of old age before we see one.
Assuming a typical saiyan generation length of 25 years, the stable population you'd need in order to get an LSSJ on average "every ten thousand years" in that situation would be, umm...
[maths]
Let's see, ten thousand years is 400 generations under that math, so each generation would need to have (one hundred trillion)/400 equals...
250,000,000,000 saiyans. Which is probably enough to overpopulate a thousand worlds, given that I suspect you can't pack saiyans onto a planet very densely before they either eat everything that grows there, blow the planet up from internal squabbles, or both.
I don't know how many saiyans you think there were during the time before Frieza blew up Planet Vegeta, but you miiiight want to knock one of those successive natural 100s (a base population of 2.5 billion saiyans will have one Big Green Machine every ten thousand years) or even two of them (a base population of 250 million saiyans will have one Big Green Machine every ten thousand years).
If it was me I'd say five successive natural 100s. If you imagine historical saiyan populations as being higher, say, more like the 20th century population of Earth, I'd recommend six successive natural 100s.