Routing through IPSec

HI,

we've got two ASGs/UTMs one is located in our central office the second one is in our branch office.
Both sites internal networks are connected with each other via IPsec tunnel.

Now the ASG in our central office is also establishing IPSec connections to some customers.

How can I make sure that clients located in the branch office network are able to connect to the customers network, using the existing IPsec connection?
I tried some settings but had no success.

Can you help me?
Is there any guide?

Thanks
jauer