It sounds like your tasks and PO's relationship is currently set up such that one task can have many PO's based on this: "in PO's I have a conditional dropdown for Tasks based on the Trade name"
Because you need to have multiple tasks for a PO, you will need tasks to be a child of PO's (one PO can have many tasks).
It would also seem like you sort of have a standard list of tasks for each standard trade, and when creating a PO, you might want to populate multiple tasks from one or more trades. (but not necessarily all the tasks listed for a trade?)
If what I described above is true, the solution you need is fairly complex and would probably require some offline help.
On thing that you could try that wouldnt require help is to create a PO tasks table, where a PO can have many PO tasks, a Trade can have many PO tasks, and a Task can have many PO tasks. On the PO task table you would now have a trade dropdown and a task dropdown (probably much like you currently have on the PO record) and you could make that conditional. Then you can add whichever Tasks you want to a PO.
Let me know if that helps. If you would like to explore other options, my contact info is in my profile.
Good Luck!
