I'm not able to reproduce the issue. I created a segment for "Visitors > 1 Country" and it works as expected. Please file a Customer Care ticket if you are not seeing the expected results & include some examples. Thanks!
BernardoC "unique visitors" is not a segmentable metric. Instead, you should choose the "Visitor" container and then build out your criteria for what types of visitors you are looking for within that container.
You can't create a segment with an embedded calculated metric, such as Approximate Count Distinct.
You could set up a cohort table with Countries on both the X and Y-axis and a metric like unique visitors. Unfortunately, this will only show visitors who saw both countries and will be incredibly clunky because there are so many countries.
Another option would be to create a visitor segment in which you require a visitor to have both a specific Countries value and notthat specific Countries value (see screenshot). If you apply this segment to the unique visitors metric you will get the number of unique visitors that were associated with both the United States and another country. You can also create a calculated metric using this segment. Unfortunately, you'll need to pick a country to start.