I don't think of it that way. If Kendrick perkins is going to the line I'm not going to say he has a 40% chance to make both. I'm going to look at it as he has a 65% chance to make the first and a 65% chance to make the second. I know i'm not making sense :facepalm
If he makes the first he still has a 65% chance of making the second. So you can throw the probablity calcs in the trash.
We agree: he has a 65% chance of making the first (or a 35% chance of missing it), and he has a 65% chance of making the second (or a 35% chance of missing it). The probability calculation is based on that very assumption.
Perhaps it's easier to understand if you look at all of the possibilities for a two-shot foul (the odds of making both in that scenario or making the first and missing the second are the same as they would be in a one-and-one situation; it's just the other outcomes from missing the first that would be different).
Possible outcomes for a 65% FT shooter:
42.25% of the time he makes both (.65*.65=.4225)
12.25% of the time he misses both (.35*.35=.1225)
22.75% of the time he misses the first and makes the second (.35*.65=.2275)
22.75% of the time he makes the first and misses the second (.65*.35=.2275)
(note that the percentages add up to 100% all possible outcomes)
In other words, there is a 42.25% chance he makes both, a 45.5% chance he makes exactly one of two (as 22.75+22.75=45.5), and a 12.25% chance he misses both.
A 65% free throw shooter makes both less than half the time. 65% isn't too far off from the NCAA average, which is usually in the high 60s. A 69% FT shooter makes both 47.61% of the time. A 37% FT shooter like Andre Drummond only makes both 13.69% of the time.
The basic point is that even the average FT shooter in college basketball (~69%) is going to make both FTs less than half of the time, so someone that's an especially poor FT shooter is even less likely to make both.