Let's say that a concave lens $A$ has a focal length of $-10$ $cm$ and another concave lens $B$ has a focal length of $-20$ $cm$ and we need to compare their powers.
We know that : $$P(D) = \dfrac{1}{f(m)}$$ Here, $P(D)$ means the power of the lens in dioptres (D) and $f(m)$ means the focal length of the lens in metres (m).
Here, $f_A=-10$ $cm$ and $f_B = -20$ $cm$.
So, $P_A = -10D$ and $P_B = -5D$.
Now, mathematically thinking, $-5 > -10$, so $P_B > P_A$ and hence $B$ should have a greater power.
But, logically thinking, lens $B$ has a focal length of $-10$ $cm$ and lens $B$ has a focal length of $-20$ $cm$. So, rays parallel to the principal axis will seem to meet $10$ $cm$ from the optical centre upon refraction from lens $A$ and $20$ $cm$ from lens $B$. So, the rays are diverging more in the case of lens $A$ since they appear to meet closer to the optical centre.
So, in my opinion, lens $A$ is more powerful than lens $A$ and we should compare the powers of concave lens by looking at the absolute values of their focal lengths and powers when comparing and negative values for magnitude and nature of the lens.
For example,
$$|P_A| = 10D \text { and }|P_B|=5D$$
$$\text {So, }|P_A| > |P_B|$$
Thus, lens $A$ is more powerful than lens $B$
Let me know if I'm right.
Thanks!