Hello,
I supposed you are using NAT since you only have two public addresses. You can specify the Guest-vlan source in an access-list and create a pool with your secondry public address.
(This is only pseudo code)
access-list 1 permit 172.16.1.0 0.0.0.255 (your guest VLAN)
ip nat pool [name] (your secondry IP) (your secondry IP)
Ip nat inside source 1 pool [name] overload
I hope I understood your question correctly.
Masoud