Hmmm... Looking at
http://wa8lmf.net/APRS_PSK63/index.htm#Background and am not sure if the configuration below will work.
"As a dual-port send/receive terminal.
The first port is the PSK/GMSK/FMSK sound card mode.
The second port can be any of a variety of external hardware- or software-based TNCs on either 1200 baud VHF or 300 baud HF. For example, a classic command-line hardware TNC (i.e. TNC2, KPC3, etc), a KISS-interface hardware TNC (i.e. TNC-X, TinyTrack4, Tracker2, etc), or another (separate) sound card software TNC. The program directly supports the KB2SCS 300/1200 baud soundcard soft TNC, the AGW Packet Engine and the new UZ7HO "Soundmodem". Currently (as of APRS Messenger Ver 3.27) the latter two are supported on receive-only. Full transmit/receive capability will be provided sometime in the near future in an updated release of APRS Messenger."