Use half-duplex or full-duplex RS-485?
How many ports do you have on each side?  If you have two ports on each device, then it's easiest to just use one for each direction.  If you have devices with only one port, you can still talk bidirectionally, but you have to take more care in timing and arbitration to avoid conflicts.

If you wanted to avoid all chance of conflict, you'd need 8 ports on your controller, 2 for each slave.  This seems like overkill, though, since your environment is polled anyway.  Without a complete understanding of your requirements, it's hard for us to make that tradeoff for you.