Amy, what you say makes perfect sense, but then TT would need to amend the language on the page where these values are detailed:
"You and [your spouse] should each have your own IRS Letter 6419. Enter the amount from box 1 of each letter, even if it's the same amount."
It makes more sense to me for TT to fix the premature rounding, rather then telling people not to exactly enter the amounts detailed on their formal IRS letters. Indeed until I receive the letters today, I put the entire amount under my name only (no rounding issues). It's only once I received the letters and then read TT's input page more carefully that this arose.
