You've already gotten some good ideas here!

The ladies are so helpful on this board.

Kathleen already shared one of them - doing DITHOR the scheduled 3 days a week, and then adding a fourth time on the fifth day, as CTC only has a 4 day plan and Beyond has a 5 day plan. This would be a happy medium for your 2 dc, as CTC has it scheduled 3 times, and Beyond 5 times, so 4 times seems like a nice in the middle way to go about doing it.
Another option would be to do it 3 days a week with the 2 of them, and just have your Beyond child reading on the other 2 days of the week. We actually did this to ease my ds into DITHOR, and it worked well. You would only get through half of the genres this way, but that's just fine.
Still another option would be to do it 5 days a week with the 2 of them, and just have your CTC child do it more often. I'm thinking this could make the writing more heavy than it needs to be though, as CTC has a nice balance in place for reading/writing requirements each day.
A final option would be just to do them separately, with your CTC child doing DITHOR 3 x a week getting through 1/2 the genres, and with your Beyond child doing DITHOR 5 x a week getting through all the genres. Sometimes it's nice to do a year like this to put focus on the younger child's reading.
DITHOR is super flexible this way, so any of these options would work well. We have enjoyed DITHOR for many, many years, and I know you will too! HTH as you ponder what's the right pacing for you and your dc
In Christ,
Julie