Step 1: Use the standard equation of trisecting lines.
The joint equation of the lines trisecting angles in the first and third quadrants is given by the equation:
\[
\sqrt{3}(x^2 + y^2) - 4xy = 0
\]
Step 2: Conclusion.
Thus, the joint equation of the lines is \( \sqrt{3} (x^2 + y^2) - 4xy = 0 \).