Can you create a dedupe rule to include individual and organisation details
The short answer is no. Dedupes for individuals and oranisations are handled separately in Civi.

I'm assuming (and feel free to correct me if I'm wrong) you've got an event booking profile for an individual which includes the current employer field.

From Manage events list. Find the event in question.
From the configure options at the RH side of the screen select configure, then online registration.

Under the heading Registration Screen you'll find links to the profiles used to gather contact details. Check the Organisation Name field to see how it is configured. Frequently this uses the current employer field from the contact record.

Soenthing like this

Civi will apply the specified dedupe rule to the new individual details and match as required.

There won't be a dedupe rule specified for the organisation so Civi will apply the Organisation- Unsupervised Rule (Default setting for any situation where members of the public are adding contact details)

From Contacts> Find and Merge Duplicate Contacts
Scroll down to the Organisation, Unsupervised rule. Edit the rule to check how it is configured. 

If your organisation- unsupervised rule is anymore complicated than match by organisation name only. Civi will decide there is not enough info to match, and then create a duplicate organisation.

If you have permissions you can edit the rule to match on the Organisation Name only field

If your matching rule is name only, you may still get duplicate organisations in a "Judean People's Front/ People's Front of Judea" scenario. As different users may record their organisation name in a non standard fashion.

Hope this helps.