Ok. Using a flawless ditto (which should not be that hard to obtain, just take some time), I believe the highest chance to get two IVs that you want down the line can be as high as 50%. Here's how I reckon this will work...

First, synchronise something that you want to breed egg moves to lock in a nature, then catch something from the friend safari that you are looking to breed with one of the IVs you are eventually looking for maxed. If you're not breeding egg moves, you can skip this step. Otherwise, breed the thing with the egg move holding an everstone and the parent with a power item (example: find an adamant aipom with baton pass and a torchic with a power anklet in 31 speed. This will create a torchic with adamant, baton pass, and 31 speed 100% of the time).

Then breed that pokemon with an everstone and a flawless ditto and the chance of specific IVs can be calculated using a fixed formula.

Ditto: 31/31/31/31/31/31

Other: x / x / x / x / x /31

This will pass the egg moves and nature 100% of the time, and the chances of getting different desired 31s, assuming the "other's" IV of 31 is a "desired IV", is...

1: 83.3% or 5/6

2: 33.33% or 1/3

3: 12.5% or 1/6

4: 4.16% or 1/24

5: 1.041% or 1/96

The chances of getting a flawless poke from one flawless and another with one 31 IV? 1/3072, or .0325% chance. Low, but a far cry from what it used to be--9*10^-8%, or 1 in a billion chance.

What I can conclude from this is that the chance to get "sweeper" IVs, as in Speed and an offensive stat of your choice, following this method, is 33%. Amazingly high. The chance for "mixed sweeper" or "wall" stats, with maxes in both offenses and Speed or HP and both defenses is still very high at 12.5%.

Why is the first stat different from the last? If you have one flawless poke and something with 31 in speed, the destiny knot will chose five out of six IVs, or 5/6. The chance of it choosing the stat where both IVs are 31 is 5/6, very high! For the next case, the chance of the destiny knot choosing two stats you want is 4/6, which looks like this:

31/31

x /31

The chances of getting double 31s here is 50%. Multiply that by 4/6 and you get 33.3%.