Investigators found the mixed DNA of Meredith and Amanda in a total of six samples among dozens that were taken. Three of these samples were from the bathroom that was shared by Amanda and Meredith. The other two housemates used a different bathroom. Mixed DNA was also found on swabs taken from Amanda's room and a latent shoe print in the hallway. A swab from Filomena's room revealed Meredith's DNA with what appears to be a very weak profile for Amanda.
All of the mixed DNA samples from the bathroom were visible bloodstains. Most likely they were composed of Meredith's blood mixed with an organic residue containing Amanda's DNA. No test was performed to determine if any of these samples contained the blood of both Meredith and Amanda, and there is no evidence that any of them did.
The other three mixed DNA samples were taken from latent stains revealed with luminol. No test was performed to confirm the presence of blood in any of these samples.
The prosecutor has tried to insinuate that these findings are incriminating. The most plausible explanation is that the mixed DNA is simply a result of cohabitation. As an example for the sake of comparison, investigators used luminol in Raffaele's apartment and found a latent stain with the mixed DNA of him and Amanda. Here is an example that you can relate to in your own home. If you cut your finger and your blood lands on a sink in a bathroom shared by other person in your house, you will get the exact same result. Your DNA will be mixed with the DNA from the other person that also used the bathroom. All it means is that two people have been sharing the same space.