Unfortunately you can not.
Without purchasing additional hardware, using the controllers to create two separate systems as you suggested would be your best course of action.
To maintain some form of redundancy should you decide to deploy the equipment this way and use a B@AP topology. I would recommend enabling; 'maintain client sessions in the event of a poll failure' and 'restart service in the absence of a controller'. This can be done by going to the advanced menu of each AP and checking the boxes.
This will ensure that the AP remains active in the event of a link loss with the controller. Please understand that AP logging, software upgrades and configuration changes will be unavailable until the link to the controller is re-established.
I hope this answers your query.