If you need to create the parent and child task when you create the account, then you need to do so in the same workflow.
Your workflow probably looks a little like this:

When you create the second task, and need to set the value of the parent task, click the lookup and in the field "Look for" on the right side, you should see your dynamic options for value.
In the bottom of that dropdown list, there is an area called "Local values". You should be able to see the value "Create parent task".
If you insert that in the lookup field it will populate the parent task

Rune Daub Senior Consultant - Dynateam CRM http://www.dynateam.dk