There is no need to end up at the city whence you began - just the country. And the U.S. and Canada are one country, according to OW.

The limit of two stopovers in the continent of origin puts some serious constraints on the utilization of those six segments. People here are always coming up with clever routings that I never thought of, but I think it works best if you can combine your rtw with a paid (or award) trip.

Thus you begin by flying to Canada on another ticket, beginning your rtw (being careful to not use a stopover unless you really get something worthwhile out of it), "completing" your rtw back to your home city, and using your first stopover. Later, you continue your rtw to use however many domestic segments are still left, using your other stopover, and ending up either back at home or someplace you want to stop and have a "paid" ticket back home from.

AA award travel works well for this since they're generous regarding the routing (allowing an open jaw, among other things, and a year to do it).