It is pretty easy to just split user stories to fit them into a single sprint but the problem is the developers, who are claiming that they can't build some stories parallel because they are modifying the same part of the user interface or piece of code in the back end. From the PO's viewpoint, those stories are important to get implemented during the sprint but that's not happening if the developers couldn't do the work. I think it is obvious that contributing to the common sprint goal will affect to same code.
Do you have any ideas to solve that kind of problem?