I have one child who has finished the Miquon / Singapore combo and another who is about halfway through, and one for whom Singapore did not work at all.
For those who did / are doing the combo, we worked through each series independently. First a book from Singapore then a book from Miquon. Sometimes we would do two books from one series then switch back to the other. I never tried to coordinate anything, we just worked through the books.
The one thing that we have done that I haven't seen mentioned, is that we sometimes switched mid-book. If a child hits a wall with a subject (subtraction, fractions, division...) we sometimes just switch to the other series. It doesn't seem to matter which series we are in, it works both ways. We just pick up where they left off (in the new series) and by the time they get to the trouble subject in that series, it is no longer a trouble subject. Or if that subject doesn't come up, I just have them finish the book and then go back to the "old book" with the trouble subject and somehow it is no longer a problem.
This may be because my children seem to fly through math books and sometimes need a few weeks to let a topic "simmer" before they really "get" it.