No, as a generic 'terminal/device server' cannot understand how to mix 2 client-server conversations.
The DOIAP can do this for Modbus or Rockwell, as it understands what is a request & the matching answer. So like a print server, it can juggle the 2 conversations at once.
For your S5, I'm assuming this is MPI (or whatever the serial encap of MPI is called), and the DOIAP has no built in intelligence to understand how to safely let two clients mix their traffic.