I believe that's not possible. You should make sure both machines use dosbox since the network is virtual and only exists within dosbox.
I think that ipx is emulated through tcp/ip. the ipxnet connection can only be emulated using a tcp/ip internet (or lan) connection and that way you can use ipx.
I could be somewhat wrong, though I'm pretty sure both computers must use dosbox in order to join the network.
Or you can search for a network driver for your card somewhere and use a real network, if that's an option for you.
I'm sure I've read a guide for this, but I can't remember where I found it. Possibly on this board, or maybe even in a readme file that comes with dosbox, can't remember.