[Exim] domain forward

hello,

i'd like to manage mail by myself without using the services of my isp.
i've got 2 visible name from internet :
boucher.ath.cx
and massot.ath.cx

I have 2 internet connexion on 2 different PC.
They each uses one of theses 2 names
These 2 PC are also on a lan,
view from the lan theres names are comploteur.creolie and jean-gibbon.creolie

i have set up my MX's so that mail for the 2 domain can be managed by
any of the 2 exim runing on the 2 connected PC.

How can i tell the exim runing on boucher.ath.cx that all the mail for
massot.ath.cx must be forwarded to jean-gibbon.creolie.
and inversaly on massot.ath.cx, mail for boucher.ath.cx must be forwarded to comploteur.creolie

Thanks a lot for your help

Edouard