Is this even possible? They seem to contradict each other in some aspects. The biggest one being emotional dysregulation.
Schizoids typically don't feel emotions very deeply, while borderlines feel them very intensely. Being SzPD is largely down to being a loner and wanting to do solitary activities; you don't even want to date anyone. From my research, you can still feel bad about not socializing sometimes if you have SzPD, and they often have a few friends they occasionally socialize with. Meanwhile, being BPD is largely down to intense fear of abandonment, emotional dysregulation, chronic feelings of emptiness with identity disturbance, and impulsivity.
What would someone with both SzPD and BPD look like? Would this be like a big introvert who doesn't really care about socializing that much, but develops an intense fear of abandonment if they fear they will lose someone? Would it be someone who doesn't go out much or tried to minimize how much they go out out of fear of judgement from others or negative, intense confrontations? Would this person be experiencing intense emotions, but at home while alone or alone in his room?