Did anyone find any real numbers for this, as far as I can work out the second double begins to cast at 46 seconds

1033427 hp per tank is 2066854 total hp.

Broke down into 4 20% phases is 516,713.5 total raid dmg needed per phase in 45 sec which is a total of 11,482 combined dps needed from your raid

Shadow and Vanguard tanks can push around 700-800dps max, guardians 400-500dps max so say we take a shadow and a vanguard that is 1400 dps of the total leaving 10082 dps needed from dps and healers so thats 2k per dps and around 500 dps needed from each healer.

Which means if you take two guardian/jug tanks your going to get two doubles every time as 2k dps is the limits of what most classes can do now other than sent / vanguard and the occasional slinger pulling 2100 ish

Feel free to add anything if I have missed anything here ?

If you put three or four dpsers hitting one tank, the dps required to avoid a second double destruction lessens significantly. At the same time, I've seen videos of groups take two DDs. So it doesn't matter either way.