On my W2, in box 12, it says: D 21 58.95 D 2040.32 Do I just add these 2 amounts and list them under D? I tried to do it separately, but it would not let me put 2 Ds.

code D is suppose to be for the amount of  elective deferrals to a 401(k) and for the amount of deferrals under a Simple retirement account that is part of a 401(k). so there should be only one number. is that box D21 (doesn't exist ) or D $2158.95 (makes more sense). but why 2 numbers?  you'll have to ask the employer.

Yes, if the D categories are listed under the same W-2, you can add them together and enter them as one entry.  If they are listed on 2 separate W-2s, you will want to list each W-2 separate.