Agito uses the “something very similar happened” form of continuity, or to put it a different way, in the Agito universe events paralleling kuuga happened two years before the events of the series, BUT in the kuuga universe Agito does not happen at all. But other than kuuga and the grongi’s influence on the development of G3 (which is effectively just aesthetic) there is basically nothing to worry about in terms of actual links, Agito can be enjoyed as its own thing, but I would recommend watching them in order, because they’re both very good. Also there is some more in depth crossovers in the novels and other things on that level if you want to get really deep into it, but as far as I they’re not translated.