There's no issue with creating a workflow on the workflow task list.  However just from a design perspective, what do you need the second workflow to do, that the first one can't accomplish?  

you definitely can run into locking issues  -  

May be you can try creating another task list for the second task. It is better if you can explain the requirement.

The main workflow generates "n" task per item that must be approved in parallel. The task has three possible answers (Approve, Reject, Request Changes) and the logic to handle the answers and control that all approvers have finished the task is in the task item workflow.

So many task are created, each task trigger a workflow when is modified to handle the answer and my concern is that the workflow can generate issues in the tasks.

Thank you all.

as already asked by Sojan Mathew‌, can you provide details what's so specific on your process requirements so that it can not be handled with main/list workflow and that you want to create a workflow on task list?

what's the exact logic task list workflow should encompass?

once we know details one can provide suggestion for other/better approach.

next I would encourage you to use community search functionality and you will definitely find plenty of other problems reported with workflows and other customizations on task list

see 3 more examples that appeared among top search results

https://community.nintex.com/docs/DOC-1307
https://community.nintex.com/message/28247-error-task-is-currently-locked-by-a-running-workflow-and-cannot-be-edited
https://community.nintex.com/message/30242

Agree, there is no general problem in using workflows in the workflow task list but you can easily run into problems locking tasks and confuse workflow. I'd suggest to try to find alternatives to manipulating anything in the workflow task list and only do it as the last option.