A biconvex lens has power $P$. It is cut into two symmetrical halves by a plane containing the principal axis. The power of one part will be:

When a lens is cut along the principal axis, the curvature of the surfaces remains the same. The focal length $f$ is given by $\frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})$.Since $R_1$ and $R_2$ do not change when cut longitudinally, the focal length and hence the power $P$ remain unchanged for each half.