You are correct in stating that the pix will block incoming requests to a inside device that does not have a static associated with it and the traffic does not match an acl entry (ace).
How does your firewall cluster work? When it sends out a packet, the ip source address should be the cluster, correct? If so, then you only need one static: static (in, out) y.y.y.y 172.16.0.25 netmask 255.255.255.255.
Otherwise you can do a nat, a global, and a static like this:
nat (in) 15 172.16.0.1 255.255.255.255
nat (in) 15 172.16.0.2 255.255.255.255
global (out) 15 y.y.y.a
static (in, out) y.y.y.b 172.16.0.25 netmask 255.255.255.255
to get both units to use the same outside address, and use another outside address to refer to the cluster ip.