Android N introduced the ability the group notifications together and with AutoNotification you can do this yourself!

STEP 1 - CREATE TASK AND FIRST NOTIFICATION

• Create a new Task in Tasker
• Add an AutoNotification action
• Set the Title to something like Testing Groups
• Set the Text to 1
• Go into the Grouping section
• Set the group key to something like some group

You can set the group key to anything you like as long as it is the same as the other notifications you want to group together

• Make sure Is Group Summary is not checked
• Accept these settings and go back to Tasker

If you would run the task now you would notice that the notification looks completely normal because there's only 1 notification in the group and there's no group summary notification

STEP 2 - CREATE SECOND NOTIFICATION IN GROUP

• Copy the notification action you just created and paste it
• Edit the second action and set the Text to 2
• Make sure your second action has the same group key as the first (it should because you copy-pasted it)
• Accept these settings and go back to Tasker

If you now run the task you'll notice that the 2 notifications are still not grouped together despite having the same Group Key. This happens because no Group Summary Notification was provided. Let's do that now

STEP 3 - CREATE GROUP SUMMARY

• Paste another copy of the AutoNotification action

By pasting the same Action you've ensured all the notifications have the same Goup Key ("some group" from Step 1), which is what designates all these notifications belong together.
Of course at anytime (and in any other Task) you can create additional notifications, and as long as they use this same group key - they will be grouped with these notifications.

• Make sure it's the last action on the list
• Edit it and change the Text to something like group

Although this text won't appear anywhere in the current Android Version it's a good idea to set it anyway in case Google decides to make it appear somewhere in the future

• In the Grouping section, enable Is Group Summary

By enabling this you're telling Android that this should be the notification that contains the other notifications in the same group. Some stuff like color and subtext can be applied to the group summary and it'll change the look of this notification but most of the fields in the AutoNotification action won't do anything to the Group Summary Notification

• Go back to Tasker

STEP 4 - ADD WAITS TO BETTER VISUALIZE

• Add some 1 second actions between the notification actions so that you can better visualize what's happening

STEP 5 - TEST

If you now run the task you'll see that the first notification will be created, then the second one will be created independently from the first and finally the group notification will be created, grouping the previous two together.