Haaland may have always been considered by Real Madrid as an alternative to Mbappe but Real Madrid's main and first target was Mbappe. Mbappé has been making deals with Real Madrid every season but the deal has not been completed but Real Madrid have waited, leaving us to wonder why Mbappé was wanted by Real Madrid at any cost. Mbappe was wanted by Real Madrid at any cost and Real Madrid waited for him. If there was no extra interest in Mbappe, Real Madrid would have completed the contract with Manchester City star Haaland a long time ago.
These two stars Haaland and Mbappe will never play for Real Madrid at the same time because Real Madrid already has the best players, so Real Madrid will definitely not want to have two finishers in the team at the same time and leave one on the bench.
I think a "never say never" is appropriate here.
I agree that it is not the most likely scenario that Mbappe and Haaland will play for the same club at the same time, but I wouldn't rule out that possibility entirely. Recently, Haaland avoided a statement where he was asked whether he can promise that he won't play for Real Madrid. He said he can't and I think it is the only reasonable answer to that question because at the end of the day, he probably doesn't even know himself what's possible in his career and what is not.
In theory I think Haaland and Mbappe could be a possibility. Imagine a team with Vinicius Junior on the left, Haaland in the middle and Mbappe on the right as a winger/striker. It would be insanely strong and I am sure that this has been thought about in Madrid already. But financially it would probably not be possible unless both players could be signed as free agents.