[Cryptography] zeromq/libsodium/elliptic question: are shared keys okay?

[Kent Borg]

Context: I am looking at using Zero MQ in a project. It has security 
features using libsodium's elliptic curve public key cryptography.

Question: Is it okay for Alice and Bob to communicate with each other 
using a single shared public/private elliptic key pair? Experimentally 
it seems to work, but does it introduce any security holes? (Beyond the 
obvious that keys can't be individually deployed and revoked when they 
are not individually issued.)

Motivation: Alice and Bob are in the same household, they trust each 
other. They are, um, liberal, Charlie might be joining, too, they will 
trust him, too. When he does join they would rather not do a bunch of 
two-way key distribution. Also, there might be more than one instance of 
Alice (and of Bob and Charlie), and the Alices (and Bobs and Charlies) 
want to be able to talk to among themselves. They are willing to rekey 
the entire household if need be. And if later they need more resolution 
of who trusts whom they can start issuing some unique keys then. But in 
the meantime, does sharing keys open up any vulnerabilities?

Thanks,

-kb