Segmentation: merge multiple PolygonLabels to one annotation

It would be nice to be able to 'merge' multiple PolygonLabels to one annotation, e.g. in Coco formate:
segmentation: [[0, 0, 1, 1, 1, 0], [2, 2, 2, 3, 3, 3]] i.e. two unconnected triangles belong to one annotation.

This is especially convenient for Instance Segmentation, where one annotation describes the mask of one instance (which does not need to be connected...)

If this is already implemented, please hint me towards some documentation - but I haven't found any on the website.