I thought a partigyle was take half of the first runnings that you would have for the second runnings beer. ie, like 5 gallons first runnings strong wort to 10 gal for a normal strength beer. The british would make a mash for really strong wort and then sparge to get the remaining sugars for a second wort.
i did one a while back where i took 10 gal first and 10 gal second and the second runnings was very low, and the first runnings were ok. So you have to reduce the first runnings to inc the gravity and then add the remaining to the second runnings to inc that gravity.