Pressing the shortcut key for attacking (default is 'A') selects/deselects the primary attack for the selected units. There is no keyboard shortcut I know of that allows for selecting the secondary attack, which is selected by clicking its icon or automatically activated by attacking (right-clicking) a unit that can be attacked by secondary but not by primary.
I propose cycling through the attack skills, so e.g. if Tech's Battle Machine is the selected unit, pressing 'A' would select, in order, Attack->Attack Arrow->no action.
If multiple units of the same type are selected, cycling should work as if only one unit is selected. If units of different types are selected, only the primary attack should be selected.