sorry, it's not fully possible at the moment in Reaper to edit multiple Midi items at once. I hope we get that at some point. see this thread for a previous discussion about the topic and what is currently possible (especially posts #4 and #23. With the fingers extension mentioned in post #4 it's also possible to quantize multiple Midi items at once with the "groove tool") edit: one simple solution for you might be to glue all the Midi items in a track and then open the new glued item in the Midi editor, but this may not be useful in all situations.