In the event that both planets of a binary planet system were life supporting, and both ended up developing a sapient, tool using species, at what technological level would they be able to start effectively talking to each other?
Now when I say "effectively communicate" I mean at what point would it be reasonable for meaningful massages to be send to the other world(the ability or the other to respond would be nice, but not needed immediately)? would this be possible prior to both species having developing Radio communications to send and receive with?
Obviously the technological levels of each species would vary due to countless reasons, and one will notice that there is a civilization on the world in the sky before they other(though cultural, religious, and other preconceptions/biases would cloud how they interpret this fact until they have really good observation technology to more closely/accurately look at their neighbors), but beyond doing things that are basically "hey! We exist! look at this giant structure we made to get your attention!" and "we know! here's visually identical one we build to acknowledge it!" which could be easily motivated by and misinterpreted through those previously mentioned biases and preconceptions of the universe, not to mention if it happened before a species had fully explored its own world(or at least the tidally locked side facing the other) then this could lead to an interplanetary game of telephone as civs build structures inspired by alien structures that were inspired by structures from another part of their own world(but that's going into the psychology and cultures of two alien races growing up next to each other, and how it effects each other over the course of their histories, which is a bit beyond what I'm asking here)
EDIT: Since someone asked for details on the worlds themselves here they are; the binary worlds are of course tidally locked to each other at a distance where they'd have an Earth-moon like view of each other, they are of similar "temperate" climates and changing seasons due to their similar position relative to their sun, though obviously the weather would be different due to the atmosphere and landscape of each world having differences(higher or shorter mountains, shallower or deeper oceans, how much surface area those oceans cover, different ratios of nitrogen, oxygen, carbon dioxide, and other gasses, and so on).
And to go into more detail on the species as well, since I have a vague idea of what I'd want them to be like, though I'm not sure if that's that relevant to the question of what tech they'd need to start properly talking to each other; the one living on the world with more oceans is a partially amphibious species, and the other is a "reptilian" species that endures the winters with a form of hibernation that induces a lucid dreaming like state of mind.