For example this sets both the X and Y of the 'Ball'.
local Ball = script.Parent while true do wait(0.2) Ball:TweenPosition(UDim2.new(0,0,0.339,0),'Out','Linear',1) end
This should keep the X static.
local Ball = script.Parent while true do wait(0.2) Ball:TweenPosition(UDim2.new(0,Ball.AbsolutePosition.X,0.339,0),'Out','Linear',1) end
This should keep the Y static.
local Ball = script.Parent while true do wait(0.2) Ball:TweenPosition(UDim2.new(0,0.339,Ball.AbsolutePosition.Y,0),'Out','Linear',1) end
I hope this is what you ment.