+1 on using Kai's calculator.
Make sure you're putting in the grain data in correctly, and after the first time you should know approximately what your conversion efficiency is (I usually get 98-99% conversion, if you're not getting >95% conversion your mashing process isn't as good as it should be, temp, time, grain crush, pH, water to grist ratio, etc).
And then you'll find you get the best results using blended runnings. Otherwise, if you just make a big beer from the first runnings and a small beer from the sparge runnings, you'll get a great big beer, but a bland and potentially tannic small beer. The small beer needs some of the first runnings. You can go the Fullers route, boil and hop each set of runnings separately and blend post-boil, or you can blend the runnings pre-boil and get pretty exact control over the gravities you want. The second is what I do, especially because it allows me (if I want) to hop each beer differently, which you cannot do with the Fullers method.
What I do is take the two beers I want, at the gravities and volumes I want, calculate gravity points, and create a grain bill big enough to reach those gravity points at the combined batch size. Say I'm going to brew a 1.090 Wee Heavy and a 1.045 Scottish 80/-, each at 5 gallons. I'd need 450 and 225 points respectively, meaning that for a combined 10 gallon batch I need 675 combined points, or a target gravity for a 10 gallon batch of 1.0675. Create your grain bill to reach that number.
I'll then do three approximately equivalent draws (drain mash, 1st batch sparge, 2nd batch sparge) between first, second and third runnings totalling the volumes that I need for combined preboil volumes. As I do 5.5 gallons each with 6.5 preboil for a 60 min boil, I collect 13 gallons between three runnings of approximately equal volume. Use Kai's calculator to figure out the volumes you need, and it'll let you know approximately the gravities you can expect (again, that's assuming you input good data, just like in BeerSmith, but it's very close for me every time). But measure each batch of runnings separately for both volume and gravity. And then again using gravity points, figure out the proportions of each set of runnings to reach the volumes/gravities you want. For me, it'll often work out to 4 gallons 1st runnings, and then 4.5 gallons from each sparge. For my mashes (the amounts you sparge, your water to grist ratio, and your conversion efficiency can easily change these numbers substantially so don't expect to duplicate, I'm just giving as an example) first runnings will often be ~20°P, the second often ~6.5°P, and the 3rd often ~2.5°P (that's ~1.083, ~1.026, and ~1.010). And then the big beer it's often something like 4:1 first, 1:4 second, and 1:1 third runnings between the big and small beer. Although again the proportions could change depending on what you want. If the two beers are closer in gravity, you may want to more evenly split the first and second runnings. If they're father apart, the big beer may get more of the 2nd runnings, and the small beer more of the 3rd. But you ultimately want to be giving the small beer at least a portion of the 1st runnings.
It'll take some mathematical trial and error (and a few batches) to figure out how this works on your system, but the point is with this method I get both beers tasting good, as well as knowing what I'm going to get before I even start brewing.
