interesting question and the answer would be more of a lenghty discussion in lieu of a simple reason.
I would think one needs to look at the History of turbine controls and its evolution to get a good feel for this type of question.
I am going to assume that your late model steam turbine has a combined casing valve with the stop being a poppet, the control being a spool and both using the "same" seat. prior to this design, these types of valve configurations were used in single casings.
Being able to condense both valves into the same casing requires though about how each of those valves operated. the stop being a positive seal cannot open with ANY steam pressure across the disk, thus it can only reset once all reheat steam has decayed where the spool design of the CV has the upper balance chamber to negate the steam forces and allows it to modulate. Now if the CV seat was inside the SV seat the two would be in interference and not be possible.
You will have to provide some information, like a cross sectional of the valves and configuration for me to continue the discussion.
Some other considerations for valve placement in the steam flow depends upon the control scheme. for example, if the unit has a "trip anticaptor" then the ICV is used to reduce pressure above the ISV so that it can reopen before the ICV during coast back to rated following a rejection.
