Wildfire will send the message to the last active user if you skip the resource. But you can the full JIDs and send the message to both JIDs to make sure that both clients receive it.
that’'s a shame - the initiator of the message has no idea on which resources the target is logged in on.
I know that some clients will allow you to send a blanket message to a group by right clicking the group name and sending message. not really what you wanted to hear, but is a good work around.