Step 1: Possible outcomes.
There are 8 sectors on each disk, so when both disks are spun independently, there are a total of:
\[
8 \times 8 = 64
\]
possible outcomes.
Step 2: Favorable outcomes.
We need the sum of the numbers on the two disks to equal 8. Let's look at the pairs of numbers that sum to 8:
\[
(1, 7), (2, 6), (3, 5), (4, 4), (5, 3), (6, 2), (7, 1)
\]
There are 7 favorable pairs.
Step 3: Probability.
The probability is the ratio of favorable outcomes to total outcomes:
\[
\frac{7}{64}
\]
Final Answer:
\[
\boxed{\frac{7}{64}}
\]