It is acceptable to have two observations of the same organism by two different observers. iNat observations are of “organism in a place at a time by an observer”, so a different observer makes it a different observation :)
You can link with the “Similar observation set” field, or alternatively use the “Observation group” field. For both of them you give them a unique value, and it could be text based eg “Darwin Frog at Chiloé”, although that might not be specific enough. The easiest way to get a unique value is to use the observation number (copy it from the end of the URL in the browser!). Then once you have it set on both observations, you can left click the field name and select “All with this field and value” to see a search page with just those observations.
[edit]: @robotpie that tag idea is pretty cool… do you find it works pretty much the same way as the “Similar observation set” does? Oh wait, I remember now, only the observer can put those on!