Berumaru soulmate au in which the last words you hear your soulmate say are written on your arm. Marco's arm had the name "Reiner!" written on it, and Bertholdt had "Wait… really? Why… are you in such a rush? We haven't… when had the chance to talk this over!" written on his.
I believe that Marco's relationship with the idea of Soulmates would be similar to his relationship to the monarchy, extremely idealized and romanticized. Deep down, he does question whether soulmates are truly meant to complete each other and make their "other half's life better" just by meeting them, but he is uncomfortable with the idea that he grew up believing a lie.
Due to this, he is a bit frustrated that his soulmate's last words to him are vague. It isn't even romantic either. The last thing he will ever hear is his soulmate yelling out the name of another person. Anyone could be his soulmate by how little there is to go off by. It didn't stop him from trying to figure out who his soulmate could be. Meeting Reiner gave him hope that they were somewhere in the 104th Cadets.
Bertholdt, on the other hand, never really believed the idea of soulmates and happy afters. He witnessed the whole thing fail time and time again. The most striking memory being him watching as Pieck and Marcel discover that not only are they soulmates, but that they will also never hear from each other again just before they board the ship to Paradis. To him, it was another thing that would be outside his control and would end in tragedy.
It doesn't help that his soulmate's last words to him imply that he was probably the one at fault. Bertholdt would read it over and over again, trying to figure out what happened and feeling sorry for whoever was unfortunate enough to have him. Whenever people talk to him about soulmates, he always shuts it down because he just can't handle the idea that he may have hurt the person who was forced to be the one for him. Not even Annie or Reiner know what his soulmate's last words are. If Bertholdt had it his way, then he never would have had a soulmate in the first place.
Then Trost happened and Bertholdt's worst fears are confirmed, that he not only hurt, but killed his soulmate. It becomes another reason why he hates himself. The story is still the same and Bertholdt dies in Shiganshina with only Armin knowing the truth that these two were soulmates.
