Well, it would have helped if you provided a little more detail, but here goes.

If the OneForAll URC-10820 has a N suffix, and not B00, it can shoot setup code Cable 1982. That should control only the DTA100. I don't know if the DRC450 has Cable 1982, but it could be worth a try.

Here in California, Comcast supplies the 1067/1167 Silver remote with the full set top box. The ones which have a red OK/Select button have Cable 1982 built in. With that remote, the setup code to use is 01982, but you need to have first unlocked the cable setup code in order to change the cable button setup code. See the 9XX command instructions, using command 982.

I expect cable 0476/1376 to control the DCT700.