With the following qmldir file:

module widgets
MyButton 1.0 MyButton.qml
MySlider 2.0 MySlider.qml

It isn't possible to use MyButton when importing widgets 2.0. However, the documentation only states "Types which are exported for a particular version are still made available if a later version is imported." (http://doc.qt.io/qt-5/qtqml-modules-qmldir.html), without mentioning any differences in behavior for major and minor versioning.