With nested RTWs you are going around the world twice (within a year). Can save a lot of money but sometimes may be better to do a paid or award round trip to get to the "cheap" RTW origin point rather than nesting with another RTW.

That said, if nest the obvious thing to do is have the most expansive RTW from the cheap origin and a less expansive from the expensive origin. Eg DONE3 ex US and AONE4 ex CAI.

Note that for One World Explorer purposes, Cairo is part of Europe so you could include it as part of your ex US DONE3 (for example).