Try using Comments.

In any of the User Action Task Forms add a Multiline textbox and config as shown below:

Comment : will be enabled to captured user's input

Previous Comments: will show all the users comments for the item on which the task is running, but the control will be disabled. This will also show the information of User Name and timestamp of when the comment was entered.

I hope this helps!

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afaik, nintex doesn't provide any support for instant messaging.

the most what you could do is to develop a customized page that will be refreshed at regular intervals.

but that will either work on whole list or single item level.

I would assume, there is M:N relation between tasks 'task requester' owns and tasks 'task worker' should respond.

but the re are no (OOTB) aggregation capabilities that would post/push updates to requester on all the tasks he owns, neither to notify/push updates to task worker on all the tasks he should respond to.

That is a good solution and probably the one I will push to my business team.

Are you using tasks ? If not, within SharePoint why not use a Multiple lines of text column and append changes. Changes are then recorded.