Xp_cmdshell Access Is Denied On Network Resource

I start receiving error 'Access is denied' executing following command:
exec xp_cmdshell 'dir \IP-ADDRESSSHARE_NAME*.*'

The configuration is following:
Two computers CO1 and CO2
Both computers are not in domain,
Both computers use the same local windows account to run their instances of SQL server 2005 and passwords for those accounts are the same
Both local accounts are in their local administrative groups
Both servers do not have configured proxy account. I execute sql commands under ther sysadmin account

If I execute this command from CO1 to CO2, everything works.
exec xp_cmdshell 'dir \CO2_IPSHARE_NAME*.*'
If I execute from CO2 to CO1 I receive access denied.
exec xp_cmdshell 'dir \CO1_IPSHARE_NAME*.*'

If I will log in to the computer CO2 under account of SQL server and execute the dir command from it, it works.
dir \CO1_IPSHARE_NAME*.*

Could you point me where to look?