I must admit that I haven't had a lot of experience with Citect but may be able to help you out. You are correct, you can implement 2 I/O servers on a single machine if using clusters. I recall that the was some Citect documentation that stated that you cannot have multiple server processes of the same type (i.e. 2 different alarm servers, 2 different I/O servers etc) running on the same PC if clustering is NOT used. So this is why the original poster would have being having difficulties. However, using clustering to solve this problem is probably not ideal. For me, clustering is a mechanism that can be primarily used for implementing process area (as in plant process) segmentation within a single Citect project (as opposed to using multiple Citect projects, i.e. one project for every process area). With clustering you can then ensure that alarms (for example) for one particular process area are not annunciated on the Citect clients associated with a different process area (and as such attached to a different cluster). There are probably other uses for clustering but this is one example. An alternative option to using clustering to solve this problem is to use an OPC server (I prefer OPC servers since the drivers are written by the actual PLC vendor) then all you need to do is setup multiple I/O Devices within the Citect project , with each corresponding to a particular OPC Server program (e.g. one I/O Device for RSLinx another for Schneider OFS etc). In this case you would still only have one I/O Server defined in the Citect project but multiple I/O devices. Hussein in your case where you are concerend that you may exceed your I/O device limit, switching to an OPC server would solve your issue. The reason being is that as far as Citect is concerned you will only have 1 I/O Device (this would be your single OPC Server) and only one I/O Server. Then all you would have to do is configure your OPC server to look at your multiple I/O devices (i.e. your PLCs etc)
Alternatively, you could use virtualisation i.e. two Windows sessions running on a single piece of hardware each with their own IP address. Hope this info helps