10 gallon batch, mash & boil longer?

I am doubling a recipe, and am curious if I have to lengthen the mash and boil times?

nope
Conversion still takes place in an hour (likely less than that but the same as a 5G batch).
Boil still takes 1 hour.

+1 on nope

Thanks

And can I still do a single batch sparge? Beersmith is telling me to do 2 sparges (Beersmith always seems to want to kick out 2 rounds of batch sparging - which I ignore and always do just one).

You can do whatever you want. It's your beer. What's your grain bill and mashtun size? Can it physically fit a single batch sparge with a 10G recipe?

Grain bill will be 21 pounds. I hope it will fit but I have not done a 10 gallon batch before, this will be my first. I use converted 15 gallon kegs for my brewpots.

After you drain the first runnings from your mashtun, measure that amount of liquid. Take that from your final pre-boil amount, and then sparge with that much.

example- I drain about 2.4G of wort out in first runnings. I want to start with 6.5G. I need to sparge with another 4.1G water, either in one or two sparges. Either way, you're rinsing the grain, and it has already absorbed water, so you are likely to not lose anything due to deadspace or absorbtion while sparging.

If you mash with a ratio of 1.25Q/lb you'll mash in with 5.5625 G of water. Lose 2.625 to grain absorption giving you a first running of ~3.9375 Gallons. Assuming you boil 1G/hr, you'll need 11-11.5G (I like an extra half gallon into fermenter to account for trub loss) into your boil pot. So 11.5-3.9375=7.5625 or just simply 7.5G need for your sparge. Plenty of room in your keg for your soaked 21lbs and an additional 7.5 Gallons of water.

How are you able to figure out the grain absorption?

Easy- do what I said in my earlier post. Find the deadspace in your mashtun, before you put any grain in. Mine is 1/2 gallon if I remember right.

Do your mash and after you drain your first runnings into your kettle, measure how much wort you collected. Add whatever value your deadspace is to the total number of gallons of wort you now have. The difference of this number from the amount of strike water you used will equal the grain absorption amount.

If you think about it, it doesn't matter much to know how much the grain absorbed, unless you're trying to figure out how much liquid+grain your mashtun can hold. The sparges will always be dependent on how much wort you have before adding sparge water, and how much wort you want to start your boil with.