You will have to remove Output 2 from Universe 2 first. Then use Add in Universe 1 to add Output 2.
In fact the limitation is on number of DMX addresses so if you have a 1 universe console you have a 512 DMX address limit and you could spread those anywhere across 64 universes (silly I realise). So you could use Output 1 with Universe 1 and Output 2 with Universe 2 if you wanted so long as you didn't have more than 512 DMX addresses patched across both Universes.
What you want to do is usually what I do - 1 universe output over two connectors - 1 output to the dimmers and 1 output to the DMX chain around the LED fixtures.
After a desk reset, Universe 2 will have Output 2 assigned:
Click on "Output 2". Then click Remove on this screen:
Universe 2 will end up looking like this:
In Universe 1, click Add then type 2 to add Output 2 to Universe 1: