The simplest is to have them run on end to end. For example this trip report.

More complicated is to have 2 (or more) running simultaneously, which might go something like this (simplified)

RTW1 europe to somewhere cheap to buy another RTW, stopover
RTW2 normal itinerary, including stopover(s) in europe then back to origin
RTW1 resume and do rest of this RTW

As there is a 1 year limit, both RTWs need to be done in a year.

You could also have 2 intertwined RTWs from the same origin, but this is more tricky and requires either other positioning tickets or careful manipulation of routing.