One thing you could do would be to collect your big beer as a 10 gallon no-sparge batch and your small beer from a batch sparge. You'll need to know your conversion efficiency for this. Once you've calculated the volume V and gravity G you need to collect to hit your desired original gravity for the big beer, you can calculate the necessary mash thickness by first converting G to degrees Plato (call this g), then:

Mash thickness (qts/lb) = .48((qts/lb)/(l/kg))*(extraction %/100)*((100/g) - 1)

If you are getting 100% conversion, your extraction % will just be the extract yield percent of your grain, typically around 80%.

Now you can calculate your grain bill weight:

M = Grain weight = (V in qts)/(mash thickness - absorption rate in qts/lb)

Absorption is typically around .1 gal/lb or .4 qts/lb.

Now you can see that this is maybe not such a great idea: Using this method, assuming 100% conversion and 80% extraction, if we want to collect 12 gallons of 1.092 SG (22 Plato) wort to boil down for a big beer, we will need a mash thickness of 1.36 qts/lb, which translates to 50 lbs. of grain. That's an awful lot, and the calculations for the small beer are even more complex...