add_library(scopedA INTERFACE IMPORTED)
set_property(TARGET scopedA PROPERTY INTERFACE_COMPILE_DEFINITIONS "SCOPED_A_1")

add_library(scopedB INTERFACE IMPORTED)
set_property(TARGET scopedB PROPERTY INTERFACE_COMPILE_DEFINITIONS "SCOPED_B_1")

add_library(scope1_iface INTERFACE)
set_property(TARGET scope1_iface PROPERTY INTERFACE_COMPILE_DEFINITIONS
  "$<TARGET_PROPERTY:scopedA,INTERFACE_COMPILE_DEFINITIONS>"
  "$<TARGET_PROPERTY:scopedB,INTERFACE_COMPILE_DEFINITIONS>"
  )
set_property(TARGET scope1_iface PROPERTY CUSTOM_PROP
  "$<TARGET_PROPERTY:scopedA,INTERFACE_COMPILE_DEFINITIONS>"
  "$<TARGET_PROPERTY:scopedB,INTERFACE_COMPILE_DEFINITIONS>"
  )