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What boiler pressure?  Is the steam being utilized for turbine equipment, or just heat?

Your ion exchange system may be able to handle nearly any output available from the EDR, but this depends on what your target final water quality actually is, and what throughput you expect to achieve from your cation and anion beds before breakthrough, and what you are using to determine breakthrough.  For example, you might base breakthrough on EC of anion bed output, or it might be silica.  You might base cation bed breakthrough on sodium level or pH increase, or even calcium, depending on the water reaching the cation bed, and what you want to do with it.

The mixed bed will obviously last longer and perform at a higher output quality if the two-bed system is handling better than 95% of the ion load in a balanced way.