I'm not sure what you mean. You want the system to know what you're supposed to do next and create a task for you?

Just type in the task, "Follow up with Janet". Task created.

What exactly do you want the system to do for you?

Duplicate and edit the reminder task before completing the parent task of prepare costings.

You could put a repeat after on the task you sent to Janet in your example. It wouldn't be able to have a different title though to my knowledge. But you could edit it after it gets created.

how about setting up a task that says:
prepare costings / send costings to Janet / call Janet re: costings

When you have the costing prepared, edit the task to delete the first step and update the due date, etc.

delete the "send costings to Janet" section what that's complete, and mark the whole task complete when you've made the followup call.

combining steps is not usually considered "good practice" and you'll lose some features, but you won't lose track of the steps.

one reco I've seen is to set dependent tasks as a tag such as #depend and set them due to an interval you can tolerate (i.e. on Friday). then hide the #depend tag in your smart lists and trust that your dependent tasks will pop up on the due dates you've set. If you're ready for them, you can then scrap the #depend tag and they'll appear on your smart lists.