Okay, this was not that hard as i expected.
- I created a custom newform in the 2nd tasklist.
- I created a custom "Add task"-Link to the New custom field in the 2nd list with a
parameter which sends the listid of the item of the first list. - I took the parameter from the custom link in the newform
- I autofilled the parameter in the lookupfield of the 2nd List and made the textfield readonly in javascript
This is only a generalized how-todo-do, beacause i think all the code i wrote is very likely to improve. If you're searching the web for the particular keywords mentioned above, you will be pretty easy autofill the field.