Hi Marco,
I'm afraid you can only add one single keepalive to a CSS service, so your only option would be writing a scripted keepalive to achieve the behavior you want.
I would suggest you to take one of the predefined scripts already doing similar to what you are trying to achieve (for example, the "ap-kal-httplist") and then modify it to your needs.
For more details on the scripting language itself, please, have a look at the link below:
http://www.cisco.com/en/US/docs/app_ntwk_services/data_center_app_services/css11500series/v7.20/configuration/advanced/guide/Scripts.html
I hope this helps
Daniel