It would be great to have an option to create grid with multiple TBODY elements.

Why? The reason is quite simple. Because of CSS styles for multiline rows.
For example, each logical row in our grid has two or three physical row elements.

Now, if you need to style whole logical row (when hovering any of physical row inside logical row), you need to have it encapsulated in some parent element, because CSS has no option for parent or previous sibiling selector.
So only way to style this is build your own grid like table without using all good from Grid.

If it was possible to add more Rows objects or some RowGroup that will generate multiple HTML elements like TBODY, then I could style whole TBODY instead of single physical row (TR element).

Maybe this could be a completely new component like MultiLineGrid.