On my board, I have two ground nets: GND and ANGND.
I want to connect them at only one point, for noise reduction reasons. In other words, everything on GND should be connected together, everything on ANGND should be connected together, and GND and ANGND connected together only at one place
How should I do that?
Brand new interactions and interfaces. 
Easy to use and quick to get started. 
If this is for PCB layout purposes then I think you will have to give them the same net name and then the Schematic and PCB editors will allow you to connect them together without an error.
You will have to mark the schematic with text to distinguish the two nets.
You can use different colours and/or widths to help with that too.
I have used the fact that you can draw angled nets to show things like a star earth connection or a Kelvin connection to a resistor.
I haven't tried but I don't think there is a way to give a net and alias so that the same net can have two different names.
If this is for simulation purposes then you can join them with a 0V voltage source. This allows you to have different netnames on either side of the source.