I did this the for my last MR, however, I only input my SQ KF number on check in (doesn't work with OLCI), it was with my MP number all along. Boarding pass was printed with my KF number and I get to keep my econ plus seating, zone 1 was valid for both programs, so I do not know which one was shown, it does show that I'm a *S also, the highest of my 2 status.

However, it took about 2 weeks for the miles to credit as opposed to the usual few days that I had experienced with my KF number there from the start.