Need to set two objects on top of each othr perfectly

[Anonymous]

I have these two objects -

I want them to sit on top of each other perfectly, to make an arch shape. I don't want one object to be a bit to the side. I want them to be connected perfectly.

I tried rotating and moving the objects manually, but no luck. Any easier way/tips/tricks to accomplish this?

[damaggio]

Just snap the corners together or create Arch with pipe primitive instead.