In strict terms, Glashuette Original is not a company, but a brand. The company's name still is "Glashuetter Uhren-Betrieb" or GUB, and this company owns the brandname GO. UNION Glashuette is not only a brandname, but also a company, which is a 100% subsidiary of the GUB. However, Union does not have a company-owned production facility. Its headquarter located in the GUB building at Glashuette, Union assigns the production of its movements to GUB (all this is from a strict organisatorial point of view). While Union has an own staff, this is not numerous. AFAIK the conceptionof marketing is done in the same office for both brands, GO and Union. However, since recently two different PR agencies have been assigned for the advertising campaigns, because the two brands had been portraied too similar.
The suppliers for dials, hands, cases, straps etc. are the same for both brands, as are the production machineries by which the movement parts are made. Differences between GO and Union movements are present, since the very expensive production phases are altered for the Union movements. While a GO movement is decorated entirely by hand, Union movements are machine-decorated. The expensive parts of the fine-adjustment mechanisms on GO movements, which need a high amount of manual work done (swan-neck adjustment, for example), are also 'denied' from the cheaper Union movements.
Of course it is a matter of synergy, that GO and Union movements are very similar in their layout. Generally, one can say that no Union movement has been developed completely independent from any GO movement. As a rule, sooner or later, most GO movements will be also used in Union watches, albeit in a somewhat simplified form.
This leads me to the very evident question of similarities with the Rolex/Tudor brands. Well, I think that GO/Union differs from that in more ways than they are similar. At least until very recently, Tudor watches did not have an individual styling, they were - to express it drastically - but cheaper 'in-house copies' of the popular, but expensive Rolex watches. The idea behind this was to give Rolex fans, who did not have enough money for the original, the opportunity to buy a very similar watch, made by the original's manufacturer. Before buying a cheap knock-off fake, the customer here could buy a very weel made product and gain from Rolex' experience and quality.
Union is something different: The customer group ideally attracted by it, should not be those interested in GOs, who cannot afford them. Union tries to reach those who want to buy one or maybe two 'good' watches in his life. These people normally enter a watch shop and are ready to spend an amount of about one month's salary on a watch they expect to last throughout their life. Until now these customers bought Omegas or IWCs, but now Union wants to become a serious alternative for those. This might sound strange to watch fans around the world, who are used to look at Switzerland for good watches. But the element of national pride should never be underestimated. It is no wonder why German watchbrands are over-represented in each election organized by German watch magazines, like the "Watch of the Year", or the "Golden Balance". The term "Made in Germany" still has a very attracting sound to it in Germany and Austria, and the experiences local dealers made, show that an IWC Mark XV very often looses when customers see the Union Navigator beside it. Not because the Union is the better watch, but because it is made in Germany.
But also from a more technical point of view, GO/Union differ from Rolex/Tudor. Rolex withholds its most important asset from Tudor: its movements, leaving Tudor supplied with ETA mass-produced movements. Union watches have their own in-house movements, derivates from GO movements, but still inhouse, which is a very important sales argument (in Germany and Austria, other German watch brands like Chronoswiss, Muehle, Sinn etc. still are 'stigmatized' because of their Swiss movements). Additionally, the Union line includes complications, which are not available in GO cases: Regulator, full calendar, GMT (however, the first two are discontiued, which I personally regret).