Step 1: Formula for apparent depth.
When the light passes through two different media, the apparent depth is given by:
\[
d_{\text{apparent}} = \frac{d}{\mu}
\]
where \( \mu \) is the refractive index of the medium.
Step 2: Combine the effects of both liquids.
Since the vessel is half-filled with two liquids, the apparent depth is influenced by both refractive indices. The formula for the apparent depth is:
\[
d_{\text{apparent}} = \frac{1}{\mu_1 + \mu_2} \, 2d
\]
Final Answer:
\[
\boxed{\frac{1}{\mu_1 + \mu_2} \, 2d}
\]