Both are important games and Barcelona should be able to win both. Both are important games but the game against Real Madrid in my personal opinion is much more important than the game against Munich. But the problem is that Barca has a bad record when facing Munich and a win will allow them to end Munich's dominance. On the other hand, the El Classico match is an important match for the club and the fans because of their rivalry. Barcelona is currently on fire. Real Madrid is also solid but they do have problems in defense. Although Barcelona is ranked 1st in the standings at the moment but I choose Real Madrid to win the match.
Well usually for important matches like this usually the coach has a strategy to overcome it and of course we don't know what is planned. If you are a coach, what will you do to face 2 important matches on the same week? generally the coach will choose the one who is called the most important and put the main players on the lineup or prepare several players in the second half, if the situation is safe the players will be saved for the next fight, anyway there are many ways.
But honestly for the upcoming El Clasico match I don't really hope for Barcelona because Real Madrid as a home team will dominate and might win the match, I did not underestimate Barcelona but that was only my opinion and based on the opportunities of each team.
It all depends on the available players. When Barcelona had its best times, they had like 15 top players and almost didn't matter whether one or the other played. Of course some can't be replaced, but still that's what it is important. How many players are injured? Who is in top condition? It's all about that. Imagine Lewandowksi or Kane would be injured. Both teams had some problems to replace their central striker, which is quite normal. But if the coach sees that everyone is on a top level mentally and physically, he will surely pick the best eleven players for both games. It#<s not the time yet to take too many actions in terms of rotation if it is not necessary or if the player is not super young and unexperienced.