OPC uses an interface between them to allow cross communication between 64 bit and 32 bit applications. There are no issues with communications between them once the connection is established.
We have seen that some OPC Clients cannot browse the OPC Server if it has a different bit compatibility however this can be easily resolved with the latest components from the OPC Foundation.
The only situation where the OPC Client and OPC Server must both be either 32 or 64 bit is when the OPC Server is running as an in-proc Process. All of the components mentioned below do not run in this mode and will work together.