Hello Gerhard,
Your Secure Login Server would be installed on AS Java, you should make the AS Java highly available as per standard HA mechanism that you would for an AS Java instance which will make your SLS HA compliant.
The implementation guide does speak about the same:
http://help.sap.com/download/sapsso/secure_login_impl_guide_en.pdf
See Section 5.8.3
Regards,
Siddhesh
