Solution:
Let the common tangent be:
$$y = mx + c$$
For curve \( y^2 = 8x \):
Condition for tangency gives:
$$mc = 2 \quad \text{(1)}$$
For curve \( xy = -1 \):
Condition for tangency gives:
$$c^2 = 4m \quad \text{(2)}$$
Substitute \( c = \frac{2}{m} \) from (1) into (2):
$$\left(\frac{2}{m}\right)^2 = 4m \Rightarrow \frac{4}{m^2} = 4m \Rightarrow m^3 = 1 \Rightarrow m = 1$$
Then, \( c = \frac{2}{1} = 2 \)
Final Answer:
$$\boxed{y = x + 2}$$