Looking through this thread, I didn't really see an answer to this question, just a couple of workaround suggestions that didn't seem to work for anyone. After many head scratching hours, I finally got my similar issue to work properly, with NO workarounds. It's a shame there's really no straightforward interview questions that address this, because I'm sure many people with multiple TRAD IRAs distribute out of only one account. Anyway, you had the right idea about using "Add Supporting Details" on the RMD page for the IRA you paid the distribution from. I listed each TRAD IRA with the amount of RMD required of each one. The program creates a total and you move to the next question (How Much of this Distro is to be applied...). I selected the "SOME of this dist..." option and the program auto-filled the amount in (same amount as previous question). From there I just checked the 1099-R and IRA Info worksheet forms to make sure they updated properly--AND THEY DID!! Hope this helps someone next year...
