When slicing for single extruder the code contains no T0/T1 so it will use the currently active extruder for extrusion, so just select second extruder and print.
Depending on your start code you might need to fix setting temperatures if the contain T0 you would need to edit them to T1 or make a slice rprofile that generates them without Tx at all, so current extruder is used.
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Depending on your start code you might need to fix setting temperatures if the contain T0 you would need to edit them to T1 or make a slice rprofile that generates them without Tx at all, so current extruder is used.