2 webhosting for a domain name

[boxster]

Hi may i know what are the pros and cons if I host a domain name on 2 different shared webhosting.

Web pages are static and have exactly same contents and hosted on shared hosting

Eg. the name servers are

1) ns1.abc.com
2) ns2.abc.com
3) ns1.xyz.com
4 )ns2.xyz.com

I do see that different DNS at various locations have are using ns for the domain name.
How is this selection based on?