The 4331 does not support any form of dedicated inter-chassis link - however it is likely to be able to meet any HA requirement you have.
However there is not enough detail in your question to give you any guidance. If I understand, you have a single link from the ISP and the ISP has a single router. Has the ISP given you a block of IPv4 public address space, or will everything be nat'd?
Will the ISP give you two ports on their router to plug your two routers into?