Pretty much this, unless I'm missing something. An earlier version of the program actually used DAGs, but I found it a little underconstrained. I got pretty excited when I discovered multitrees, as it seemed to be exactly what I needed the whole time.
[x] Task
├──[x] Sub-task (1)
│ ├──[x] Sub-sub-task (2)
│ └──[x] Sub-sub-task
├──[x] Sub-task (3)
│ ├──[x] Sub-sub-task (2)
│ └──[x] Sub-sub-task
└──[x] Sub-task