Host Mail server in two Hosting Cmpanies

? We have domain name A1.com and we are registered through the company name C1.
We have a two hosting companies name A2000 & A2001 but currently our web site and mail server host on A2000.
Now we want to utilize both of them. We need to set up tohost specially our mail server on both companies, because if A2000 have some problems then we will retrieve our mails through A2001. If A2001 have some problem then we will retrieve mails from A2000.

We are running Exchange Server 5.5 (SP3) on NT4.0 (SP5) Server. We don?t have a DNS.