How do I join two RGB ledPanel 32x64 in order to get one 64x64?
First off, connecting the 2nd panel to the same output pins as the first one, you are simply sending same data to the 2nd panel, which is why you get the duplicate visuals. I'm guessing you are sending power that same way, which is why you are seeing the brightness dim.

Secondly, it's hard to read on the pics in the AliExpress listing, but the Sparkfun site had a better image, which shows an "Output" connection on the back of the panels.  You need to daisy chain the 2nd panel to the first panel through this "Output" connection. This should prevent you from having to create a 2nd matrix2 and use a 2nd set of pins.

Third, having a 2nd set of pins and controlling the 2nd LED matrix can make updating the image faster. It depends on your library as well as how the LED matrix is made, so it really depends on those factors, but some LED systems are designed so there is a maximum amount of pixels that can be displayed before there's a noticeable delay in LED updated. With this amount of LED pixels, you're probably fine, but if you want fast updates and expect to expand your matrix in the future, you might want to consider driving the panels separately. I'm not saying you have to now, but if you're seeing lag, this might be a good option.