Need advice on a split batch

Ok.. here's what I'm trying to do:

I want to make 2 beers from the same grain bill. One, a 5 gallon batch of wheat ale (haven't decided if it will be a hefe, american, etc.), and the other a 3 gallon batch of Berliner Weisse. Normally, this would be easy, but the intended OG of my Berliner Weisse will be about .15 - .2 gravity points lower than the wheat ale (~1.035ish for the Berliner, ~1.050ish for the wheat ale). I want to maximize my brew day and pull these both off on the same day, but I'm having trouble definitively picturing how I would do this.

The issue is with the sparge - would I run off the first runnings (high SG) into one BK (for the wheat ale), and then once I hit about 1.040ish start running into a second BK (for the Berliner Weisse) until I hit ~3.5 gallons of wort? This sounds... doable but very inaccurate, and I can see myself completely missing my target OGs and then having to mix the 2 worts for a long time until I hit the targets..

Has anybody out there done something similar that could give me some advice to achieve my goal?

i'd shoot for over my OG's and then add water to hit my OG realizing that it will affect your starting volumes a bit so you may have to do some math for any hop additions.

Yeah this would be really easy actually. BW have hops under 10 IBU's; however, traditionally this is mash hopped, so do a 1.050 brew that is mash hopped. You will need to water down for the BW so do as slakwhere said and figure your watering down ratio and mash hop accordingly. You will then have two different kettles with your two different worts. Do your no boil or whatever for the BW and your normal boil for the hefe adding additional hops as needed.

you could try a party-gyle.. download the simulator and play around with it..
http://www.braukaiser.com/wiki/index.php/Batch_Sparge_and_Party_Gyle_Simulator
it doesnt show a 1st&3rd runoff combination but your second runoff could be your berliner... oh, i use brewing software to dummy a recipe with my OG and preboil volume to find my ibu's when i party-gyle.. the simulator is pretty accurate but i have added DME to hit my gravity sometimes..

edmanster - this is what I was looking for! I'm gonna play around with this - I'd prefer not to water the wort down if I can avoid it. Thanks!