mail routing upon left part address

[loloyy]

hello,
i am trying to handle this on a mail relay smtp sendmail gateway.
This gateway have "default" route to another internal server in its mailertable
.mydomain.com smtp:[192.168.0.2]

now, i want to route some precise addresses to another gw

it could be something inspired with same syntax:

user-a smtp:smtp2
user-b smtp:smtp2

i tried it with virtualuser db:
user-a@mydomain.com user-a@smtp2.mydomain.com

but it does not work.

any help ?