The dollar amounts on the single Form 1099-R must be split between the two split Forms 1099-R such that the dollar amounts sum to the amounts on the original Form. Assuming that any dollar amount in box 5 is less that the amount deposited into the Roth IRA, all of the amount in box 5 and any amount in box 2a would be allocated to the one that you report as having been moved to a Roth IRA.
