How do I isolate an open collector MISO from a non-open collector MISO?
The pull up resistor will appear as a load to either of the MISO pins when they are actively driving low.  Obvoiusly, the open-collector output is designed to drive the pull up resistor when it pulls the line low.  

It is quite possible that the non-open-collector is also capable of driving the resistor.  When the output is driving high, there should be no issues, as the ouput will be at a level similar to the logic supply voltage, so the current through the resistor will be negligible.  When the output is driven low, the current through the resistor is determined by the supply voltage and the resistor value so that i = v/R, where v ie the supply voltage and R is the resistor value.  (I'm neglecting here, the fact that the low driven voltage will not be exactly zero.) 

So, you should check to see if the output can drive the current needed by your pull-up resistor.  This specification is commonly refered to as output low current, or IOL (the OL is often shown as a subscript).  If so, you should be fine.

You can adjust the value of the pull-up if necessary.  Higher value resistors will draw less power, lower resistor values will speed up the transition from low to high logic level for the open-collecotr driver.  Keep in mind that the pull up resistor will only draw current when one of the outputs is driving low, which is only when one os the CS pins is pulled low and when a low level bit is being transmitted.