Hi Rafael - I had update my reply as multi mapping without BPM is not possible for ABAP proxies/IDOC's.
is it not possible for you to have one common structure for both the interfaces?
Define one structure having two fields in it.
MT_Out MT_Inb
XML_Version Input_1
Field_2 Input_2
Field_3 Input_3
Field_88 Input_x
Field_99 input_y
And handle all the logic based on the XML_VERSION value in the mapping.