It can be done by setting the target property to "400, 0" (to tween only x, 400 pixel right) and setting the target mode to relative.
If you want to tween only Y, 100 pixel up, you can set target property to "0, -100" and set the target mode to relative.
If you need absolute coordinate (i.e: move sprite to Y: 340, regardless of current Y), that would be a little trickier. You can use this in the event sheet:
Event: On Sprite created
Action: Set Target, target is Y (Absolute), and set value to 340
Action: Set Target, target is X (Relative), and set value to 0
However, I think I will reconsider adding that 'only X' and 'only Y' to tweened property, along with ping pong play mode due to alot of demand.