Yeah, I don’t know how you would prevent 1-to-many simultaneous visits within the data model. But the concepts of visit and encounter were initially an attempt to describe “real life” clinical settings. IE, a person can often times interact (have an encounter with) multiple health workers during a specific visit (contiguous period of time where a person goes to a location 
).
So, at some level the data model has to be opinionated about such things… and this was the choice we made (and validated against HL7 v2’s conventions) when we built the data model back in 2004-2005.