I found the grit at Tractor Supply or Valley Ag, so your local feed store should have it, I know you can find it online as well. I had a spare little chick feeder so I just put the grit in it, but any small dish would be fine. They instinctively know when they need grit. Its usually white (finely crushed granite) and the chick starter is brown. I just left it in a corner and they would eat a few pieces if they needed it. Here's a link to what I have used. (I'm not familiar with using sand, so I can't make a comment one way or the other on that).
Chicks grow very quickly, you will be surprised. Generally, the rule of thumb is to have 4sqft for each bird in the coop when fully grown, at least 1sqft of roosting space for each bird as well, and 10sqft. each of range space. Now this is just a generalization so... Some people have success with smaller spaces, but the more room you have the less potential for bad behavior to develop (feather picking, fighting, etc.). The bigger the better. If you need to split the chicks between the two brooders, you may want to make sure the two groups can see each other at least, this will help them be familiar with one another when you put them all into their final destination. This might help with integration problems. The only problem I foresee is you will have two groups and each group will have an established pecking order (yes, even young ones do this), then you will be trying to merge two groups.
If you have the coop built before they arrive, and these are your first batch, you may want to consider brooding in the coop itself. There are two articles from BYC members that have a great idea for brooding in the coop. I wish that I had seen this concept and used either one instead of hanging a lamp and worrying myself to death, but there's always next time
@Blooie has an article here: http://www.backyardchickens.com/a/brooding-with-mama-heating-pad
@aart has an article here: http://www.backyardchickens.com/a/pseudo-brooder-heater-plate
How big is your coop going to be? Are you getting bantams or large fowl? Would love to see pictures of your brooders and the start of your coop.
Edited by Wyorp Rock - 12/24/15 at 6:18pm