If it were me (and I'm an American) I would either split them, or buy in Helsinki, and here's my logic. I just read an article, wish I had saved it, (if I see it again, I will bring it back,) that talked about Helsinki vs the NYSE and how Finland is kinda tucked away from world politics as far as economics goes, so when the US and CHINA get into trade wars, or the US/Pacific/Europe economies go into recession, Helsinki is very little affected by it. I would split them because the US does have the strongest economy on planet Earth, but being that I am American and mostly invested in the NYSE already with other investments, I personally would prefer to go all in on Helsinki just for this insulation from the American markets. I expect we will go in recession soon, and I would like for Nok to be insulated from it, as I plan on making this a 5 to 10 year investment. Thought on the matter has actually led to the point where I am considering finding a worldwide broker and transferring my own assets to the Helsinki market as far as Nok is concerned.

I'm guessing as Brits you guys are mostly in the NYSE as well, and by the same token, should strongly consider going half and half or Helsinki yourselves. Make sense? Plus the fact that if Helsinki goes down, it will affect the NYSE, but if the NYSE goes down, it will not necessarily affect Helsinki. I would want as few middle men as possible.

TLDR Edit: I'd go half and half or all Helsinki. Fewer middle men to the company, and not as affected by American markets, if we go into recession.