In practical scenarios, a T.38 fax call has at least part of the call being carried over PSTN, although this is not required by the T.38 definition, and two T.38 devices can send faxes to each other.
In practical settings, T.38 fax calls often incorporate elements of the PSTN network, even though it's not a requirement according to the T.38 standard.  Two T.38 devices can send faxes to each other independently.