Responses
My theory is that they both lost too much blood in the accident and the other alex's injuries were too severe to save him and in order to save mc alex they had to use other alex's blood for a blood transfusion. Which is why mc alex smells like other alex and why kline asked where he got other alex's blood from

Spoiler warning
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I wish the author left the other alex alive instead of killing him off because how would mc alex feel knowing that he was basically fall back from the other alex and if the other alex was still alive Kline would have probably tried to make things work with him. I wish the author went the route of Kline and mc alex fall in love and Kline meets old alex and they just become friends or sumn. Makes me a little sad that they never really meet old alex and Kline never really addresses that he loves mc alex more(I know he kind of does in the dream but still)