I agree that with time it may be possible, I am definitely not certain. But I would say based on what you've presented that:
a) You quoted two high profile matches in which the opposition did not sit back, so Gundogan and Kevin's counter-attacking passing was probably very good in those games. Against opposition that sit back and defend I think it's less effective to have both. Some people have even noted that last 2 seasons Gundogan has been good in "BIG" games as others have noted, but bad against the parked bus.
b) Other than those two games, and maybe 1 or 2 other games (that I've seen the pair work badly), I think in general when they have played together it hasn't been that great. Somebody will have to look up the times they have played together but I think more often than not it doesn't work out, or the correlation is it works in 'BIG' games only.
That first point is definitely an interesting point of view I hadn't considered. I do think it's somewhat true, but as a compromise I'd say there were plenty of times in those two fixtures where we were completely on the front foot and they both did well during those periods.
The second point though is probably true though. I wouldn't have them down as the perfect pairing true, but personally I do think there's potential for it to improve. I'd like to point out KDB and Bernardo so far haven't looked great together in the middle but I reckon those two will learn to mix it up brilliantly in time.
Just so you're aware of my stance, as I've only been contradicting you so far; I think us having four fit and firing CMs will actually benefit us the most. We did really well earlier this season when the Silvas and Gundogan were being rotated and I think that it meant they all developed together as pairings, kept fit and kept competitive. I genuinely believe that it can work.
I think currently there are two issues:
1) KDB isn't up to speed yet, and it's showing. He hasn't really looked too great in any pairing yet bar some obvious moments of class. And only David Silva really has the ability to manage that kind of extra workload in the middle, and that's only because he's phenomenal.
2) I have to admit, it seems like every game Gundogan is asked to do something different. One game he's deep helping distribute play. The next he's supporting the fullbacks. The next he's a creative player, making runs into the area. Then back to deep. Then he's almost a support striker. Etc etc. And when you think about it, I think he's the only one of our midfield who can really do that; two years ago not one of KDB, David Silva or Bernardo Silva were really playing in the centre midifeld position. And if you've ever seen my other posts you'll have seen how long I think it takes for any player to adapt to such a role. In fact I think the three are all still doing so to some degree. Which means Gundogan is the versatile one.
However, I don't want to make it seem like I'm coming up with TOO many excuses for him. On the one hand I think he's a lot better than people give him credit for, and he's a particularly clever player off the ball as far as I can see. For example, people might say otherwise but I don't think it was an accident he was racking up those passing statistics last March and of recent in the deeper role (same can be said about Zinchenko, not relevant but I wanted a chance to bring it up). And he has a habit of scoring in key games, and playing well in bigger fixtures, which says he has potentially big value.
But at the same time, it's increasingly feeling like we're living off potential. I will admit I wrongly said last season he would displace one of our CMs at some point but that's just how good he can be. But he hasn't been that level of good, not regularly enough at least. It's still a good enough level for me and others, but I can see why it's not for others. Taking into account how good he CAN be combined with apparent irresoluteness in the tackle and I can understand the frustration, even if I disagree.
Maybe I'll be wrong and it just won't quite work. Maybe he's as bad as any other midfielder in history as some on here seemingly insinuate. But I'll try and be optimistic about it. Because why not.