They can not. They are two different protocols, and I am fairly certain that there are no protocol translation devices.
You can configure the hosts to use both protocols, but they will only communicate with another device using one or the other protocol, not one on IPX and one on IP.
If possible you should migrate the IPX devices to use IP, as this is now the de facto standard for interoperability. Any device should have an IP protocol suite available, so this should not be too bad to achieve.
Probably not the answer you were looking for, but it <b>is</b> the answer.