"Multi-mash" for High OG in 5gal MLT?

Instead of buying a larger MLT (due to the usual constraints of cost, storage marital disputes over additional equipment purchases, etc) would it be possible to do the following in order to brew a high OG beer with only a 5gal cooler?:
-split a large grain bill into two parts
-mash one half in a 5gal MLT, sparge (limited or no infusions to limit diluting the wort)
-dump grain
-mash second half in the same manner
-combine to form a higher OG wort?

The issues I foresee are:
-calculating grain to water ratios needed to end up with correct combined OG
-lowering efficiency (dramatically?) since you are only taking first runnings
-compensating for lower efficiency = buying more grain which will eventually cost more than a 10gallon cooler?

Any thoughts?
Thanks
-ted

check this link out to see how much grain will fit in your MT with your mash thickness. Another option is using extract and since it is a high gravity beer you wouldnt be able to tell. Also that link says you max out at 12lbs at 1.25 thickness which will take about 4.71g. 12lbs x 36 (yield of 2-row) = 432 / post boil volume (5.5g) = 78 so your OG will be 1.078 or 1.062 with 80% eff and if you want it higher just add a pound of DME which will add about 45 points so your OG will be 1.086 or 1.070 with 80% eff, 3lbs of DME will get you at 1.090. How high of a gravity beer do you want.

You can also just make a 3g batch with 80% eff OG will be 1.115 12lbs at 1.25 thickness

http://www.rackers.org/calcs.shtml

A sequential mash will work, but it takes twice as long.
You could split the mash and do half in the kettle or an Ale pail wrapped in a blanket..

What hurts most is dumping the grain that has so much sugar left. I'd mash 14 lbs of grain, sparge until you get the max preboil you can handle and then stretch the OG via DME.

kryolla - thanks, i have used the rackers site before as i have been trying to stretch my MLT by thickening the mash, but have started reaching my limits in the 60's. really just want the ability to do an IIPA maybe once a year, so supplementing with DME sounds like the best route.

HairyDogBrewing - thanks as well, i figured it would take longer, sounds like it is actually possible

and Bobby_M - thats kind of what i was expecting, although now that i think about it, it seems it may be possible to simply try the "english sparge" method and i will end up brewing 2x 5 gallon batches, one heavy, one light. Am i overlooking something fundamental in that regards (other than i am going to spend a whole day in the garage!)?

Partigygle (sp) sounds good just make sure you dont oversparge, keep an eye on ph and gravity