multicast packets do not get through point-to-point interfac

We have two nodes connected via a point-to-point link. IP addresses are
configured but ‘neighbor’ addresses are not ( The nodes do not have
apriory knowledge what address the neighbor will be configured with. It
seems natural to discover neighbors address using multicast. But IP stack
(or ppp manager?) discards packets if packets’source address does not match
the neighbor address which is It is kind of chicken-egg situation:
in order to discover neighbor’s address, you have to know that address. Is
any way to disable multicast packets discarding?