And you won't.
The situation you describe is so unusual that there are no large numbers of
patients on which to base stats

Hello Hans, good to hear from you again.
Keep in touch,
J

While not particularly repeatable or reliable, just for discussion
purposes you could multiply the probability of the first with the
probability of the second.

Think 6 sided die ---

The probability of rolling a 1 is 1/6.
The probability of rolling a 1 followed by rolling another 1 is 1/6 *
1/6 or 1/36.

The problem with accurately estimating the probability you speak of is
that we cannot readily estimate the sample space - that is, we don't
know all possible outcomes, so we can't _really_ guess the chance of
any given combination.  The other concern is we don't know if there
are percipitating conditions.  Maybe you had a certain kind of
exposure a particular number of years ago that initiated both -- so
then the probabilities would be calculated differently as they are no
longer independent events.

Think of a 2 engine aircraft.  If the chance of one engine failing is
1 in 10 then the chance of both engines failing together is 1 in 100
--- unless of course the first engine failed because there is no more
fuel --- see what I mean on the common precipitating event?

Hope this helps a bit.