tying is a good option too, perhaps you can both sit together and do it.
What you have to remember is they really need to be only two or three inches apart. I've seen some (and repaired some) that just tied each corner. There is a Whole lot of shifting that goes on in a tied quilt, in use and in laundering.
be sure to use square knots. I personal favorite is DMC balls, either #8 or #5.doubled.