crossed networks

Two ISP’s with two blocks of Static ip's
One block enters building, is split by switch, and then goes into a series of routers, then into internal network to web servers, PC’s and proprietary comm. devices
Second block using one ip into own router, then into other devices inside.

*These networks then shared by unmanaged internal switches (not my doing) *

Symptoms: 1) internal computer will periodically pick up different external IP addresses (using IP Chicken)
2) Internal comm. device often addressable on 123.xxx.xx.44 drops and can only be seen on 456.xxx.xxx.67 ; it must be seen by 123.xxx.xx.44 or we lose customers.
I am able to ping from one network to other.
I'm not at all sure if these symptoms are caused by the same issue.

This crossed system was seen by client as a means of redundancy.
I assume I must disable any internal communication between these networks ?

We would need to see more about this network; however, it looks to me like this is all screwed up and you need to start at the router and work your way down fixing things. Redundancy is not a cheap thing to have in networking and rigging it like this will not work.

Honestly if I were being tasked with helping you out on this network I would need a detailed up to date drawing or a site visit to see it. This looks all kind of messed up.

If you are getting a different IP each time you visit ipchicken.com (or just random) from the same machine on the same subnet then it sounds to me like you have some sort of NAT address shifting setup and you aren't doing what we call in the Ciso world "Overloading"