Using beersmith you have two variables

1) You need to determine the total amount of water needed.

2) You need to determine the strike temperature for mash 1 using the total amount of water.

3) Efficiency should be roughly equivalent to using all the grain at once, which can be estimated using something like braukaisers lauter efficiency formulas, or my calculator (google my username).

Given the limitations of existing brewing software, no way to add the grains at specific mash steps, I would do the following.

Create recipe 1 with 100% of the grains to determine the total amount of water needed (or use my calculator).

Now remove the amount of grains you plan on using in the second mash, adjust the mash tun losses until you the total amount of water correct as above. You now have the strike temperature, write this down somewhere else, or put it in the note section or something.

Set the efficiency according to what the estimator says (not what beersmith says, because it doesn't care about the efficiency curve that dictates efficiency drops non linearly as OG increases).

Now add the grains back in, you should get a rough idea of the OG.