What you are saying is true because it has happened to Arsenal last season when they lost the opportunity they had on wining the premier League last season to Manchester City and they almost repeated the same thing again this season if not that they were able to regain there performance when the league is still on, however I don't think Arsenal playing on the two leagues will affect them because right now there focus is not really on the champions League because they know fully well that there are more quality clubs on the race for that trophy, so there interest now is on the premier League.
Most likely, clubs will always give priority to the Champions League, but the Premier League is also a very respected championship and I would not be surprised if Arteta wants to become the champion of England and sacrifices the Champions League for this. Still, there is Real Madrid, with whom it is very difficult to compete, although you can of course try to win both tournaments, but Arteta understands how difficult it is, even Bayern, which now has a lot of problems, looks like a very formidable opponent.
Yes, the point is one of the two leagues, in fact it is clear that any club must want to win many titles in various league competitions but it is not as easy as turning the palm, and yes for Arsenal have managed to occupy the first position in group B UCL but of course their journey is still quite long even though it does not mean it is impossible to win the two leagues but it is very difficult to divide the balance in the middle of the two leagues that run at the same time. In my opinion, it is better for Arteta to focus more on the English league at the expense of the UCL, because now they have managed to occupy the first position in the English league standings, although yes statistically they have the same number of points as Liverpool and also only have one point difference with the champions last season, Manchester City but at least now they only have to maintain their superiority by trying to secure their last few matches towards the end of the season, on the other hand I am sure that Pep Guardiola will not let all that happen which means Arteta's task is not easy even though he only focuses on winning the premier league.