split DNS?

[Sky Me]

Hello list,

Given the case there're two ISPs exist,and some clients belong to ISP
A,some clients belong to ISP B.Accessing from ISP A to ISP B is very
slow,then we need to put services on both ISPs.Given the server host
ip on ISP A is: 11.11.11.11,and the server host ip on ISP B is:
22.22.22.22,surely they run the same service.

Ok when clients in ISP A access the service (given the service name is
foo.example.com),their requests would be brought to
11.11.11.11.Otherwise when clients in ISP B access the same
service,they would go to 22.22.22.22.

How to do it?The way currently I used is:

Set a CNAME for 'foo.example.com' to 'foo.split.example.com' in main
DNS.Here 'split.example.com' is a seperate domain,it has its own NS
server,which is different from main DNS,let me call it split-dns.When
querying for 'foo.split.example.com' reach split-dns,it answer
different CNAME results based on which ISPs clients' IPs belong to.

For example,if client's IP is ISP A's,then split-dns answer the result:

ispa.foo.example.com

if client's IP is ISP B's,it answer the result:

ispb.foo.example.com

Then in main DNS,'ispa.foo.exampe.com' would finally be pointed to
11.11.11.11,and 'ispb.foo.example.com' would finally be pointed to
22.22.22.22.

The whole process for ISP A is:

foo.example.com  CNAME  foo.split.example.com  #1
foo.split.example.com  CNAME  ispa.foo.example.com  #2
ispa.foo.example.com  A  11.11.11.11  #3

The whole process for ISP B is:

foo.example.com  CNAME  foo.split.example.com  #1
foo.split.example.com  CNAME  ispb.foo.example.com  #2
ispb.foo.example.com  A  22.22.22.22  #3

The #1 and #3 were happend on main DNS,the #2 was happened on split-dns.

For main DNS we use BIND 9,the split-dns was written by me,based on
Stanford::DNSserver framework.All these work properly.

My question is, is this the best way?Is there any way can do it using
BIND's own features?Thanks guys.

--Sky