I would tell them to forget it.  They can set up another username (as
Nathan already described) or both read the same message.  If they are
both using the same account, I can't see any reason why they need two
copies of the message.  It certainly shouldn't be a problem for one to
not delete things before the other reads them...and if it is, it's not
YOUR problem.  Setting them up through a gateway, or multiple subscriptions,
etc, will only cause undue traffic and massive problems when/if one or both
wants to sign off.
 
My $.02,
nate