What is the overall capacity of two cascaded BSC channel?
No, this is not correct. Consider the chain of two BSCs with error probabilities $p_1$, $p_2$ as a single BSC with unknown error probability $p$. 

Now, we know that in overall no error occurs, when:


neither BSC1 nor BSC2 create an error: $(1-p_1)(1-p_2)$
both channels make an error: $p_1p_2$.


Hence the overall probability of an error is 

$$ p= 1-[(1-p_1)(1-p_2)+p_1p_2]=p_1+p_2-2p_1p_2 $$

which is also known as the binary convolution. Now, the capacity of the chain-BSC is given by 
$$C=1-H(p)$$ 
where $H(p)$ is the binary entropy function.