Mixing two blood samples from two different people would produce mixed results depending on the blood type. First, it is unlikely that blood in such a circumstance would mix together. It would probably mostly be in separate pools with some overlap. The overlap might be coagulated or contain the lysed cells from reacting with other cells. If the blood types were similar (and no lysis occurred), you would find antibodies from both persons in the pool possibly appearing as a huge string of antibodies in a single sample.
What forensic experts look at is not blood types (A, B, AB, O) but the DNA of the cells. They would quickly see that there is DNA from two or more people in the blood sample. Each person has their own unique DNA and there is no way confusion would arise from mixed blood samples.
RN/ law student