John
Yes you should be able to use the same ASN if you are advertising different subnets.
Couple of points -
1) do you have a direct connection between the existing and new sites and as you proposing to use IBGP between the two or looking for each site to act as failover for the other
2) regardless of the above if you need the existing and new site to accept each others advertisements then you would need to use the "allowas-in" command for it work otherwise because both sites are using the same ASN then they would reject the route advertisements to each other because they see their own AS in the path.
Jon
