Make sure your position assist toolbar is showing.
View->Customise->Toolbars->tick position assist tool bar.
Highlight all the buttons and press the vertical align center button on the tool bar.
Then press the equal horizontal spacing button on the toolbar.
I tried what you said, didn;t result what i want......... but you inspired me with a good workaround
I remove all the horizontal spacing in the two groups, then make them centered in the pop up page horizontally , now they are vertically centered together, then i move the whole block to wherever i want.
But i wonder is there another way??