1. When there are independent premises within a single step -- that is, when two or more arrows converge on the same conclusion -- the argument is at least as strong as the strongest component.
In one of his essays, Carl Sagan makes the following argument
(numbers have been inserted):
"(1) Science is based on experiment, (2) on a willingness to
challenge old dogma, (3) on an openness to see the universe as it
really is. Accordingly, (4) science sometimes requires
courage." (Broca's Brain)
The conclusion here is obviously 4, and 1 through 3 appear to be
independent premises for it. What is the connection between
courage and doing experiments? This argument seems extremely
weak. Challenging old dogmas could require courage, so the
connection between 2 and 4 seems stronger. Finally, what is the
connection between "an openness to see the universe as it really
is" and courage? Once again, the link with courage is weak.
Here we have two very weak links and a fairly strong one. This
argument is at least as strong as its strongest link.