Select multiple ui elements from an apple script

I´m trying to automate a workflow within FCPX with an apple script using UI access via UI Elements.

Now I´m trying to select multiple rows from a table.

Selecting single rows works fine in the forms

select row n of ... of window "Final Cut Pro"

set value of attribute "AXSelected ofrow n of ... of window "Final Cut Pro"to "1"

work fine I just can´t select more than one row. Selecting an new row deselects the others.

Is there any way to solve this?