2 domain names for 1 site

I have 2 domain names on my site - https://ligocreative.com is the new one, https://lilanigoonesena.com is the old one. Both URLs will remain as I slowly transition my business to the new one. 

But when I go to create a new property for https://ligocreative.com on Google Analytics, it asks me to put the Google Tag Manager code into the External API Keys but the old one is there. Do I replace it? Leave it? Use both? I want GA to track both URLs, is that possible?

Thanks 

I would replace the current tag with the new one. Having two domains connected, you won't get good data page to page, because of the way URL mappings work in Squarespace. It will still capture the original domain as a "referring URL," though.