4 pix firewall site to site vpn with site a 2 isp to site b 2 isp possible

I am giving task to find a solution to site to site vpn with 2 pix 2 isp site a
and 2 pix 2 isp site b.
is it possible to configure redundancy incase one isp fail with out buying the expensive failover pix.
i have 2 506e, 1 501 and 1 515e
any thoughts will most appriciated.