Multiple Domain Plugins

I’ve recently built a WP site which has a jQuery language switcher to switch between English and Spanish content.

My client has a Spanish domain and that currently forwards to the default (English) domain. However she is keen to have it so that the Spanish translations automatically load when the Spanish domain is visited as opposed to the English one.

I said to her I should be able to take care of it if her IT guy sets up the domain so that it stays in the browser’s address bar when browsing the site.

So basically I’m after a domain that will let us run 2 domains on one WP site. I’ve found these 3 but none have been updated within the last two years which puts me off a bit:
– https://www.ads-software.com/extend/plugins/multiple-domains-with-analytics/
– https://www.ads-software.com/extend/plugins/multidomain/
– https://www.ads-software.com/extend/plugins/domain-mirror/

Does anyone know of any similar / more recent plugins that will achieve the same thing?

Also, is what I’ve heard about this being bad for SEO true?

Thanks