So you have had an ‘ah ha’ moment. 
Seriously though, this was exactly what I was referring to - each part can only be edited in one sketch plan at a time, with additional parts needing a new sketch plane for each edit. Not ideal, but it’s how the system works.
Start a ‘toy’ project where you attempt to divide a part like this:
Pick any one of the two lines, to divide the original box into two, then pick any other, attempt to intersect it with the result of the two parts, and use a filter by bool mask to remove the part that doesn’t intersect. Once you have that done you can attempt to divide the included part by the line.
If you make that work, try a slightly harder problem, and then a still slightly harder one, and then try your real problem. If you can’t make that work, share your DYN and toy RVT files so we can help. 