How do you calculate the random input referred offset of a basic two-stage op-amp?
Take a look at the example circuit here.

To get an approximation of the voltage offset error at the output, look at the two equations for error due to Ib+ and Ib- (I'm on a tablet right now so I won't repeat them here.) Add the values in your datasheet for max input bias current and input offset current and plug them into each equation, and find the larger result, call it A. Plug the max value of input bias current into the other equation,  call it B.  Compute the voltage offset error at the output by multiplying the max offset voltage by the op amps noise gain.  Then add the sum of squares of the offset sources, I.e. \$\sqrt{(A - B)^2 + C^2}\$. 

This wil give you an approximation of the offset voltage at the first op amps output. You can calculate the offset at the output of the second in a similar but somewhat more complex fashion, but for a 2 stage op amp where each stage has similar gain >> 1 it is easy to see that the contribution from the first stage will completely dominate, and the offset at the output of the second stage will to first order just be the offset of the first stage multiplied by the gain of the second.