Al-Sha'bi (may Allah have mercy on him) says that Qadi Shurayh (may Allah have mercy on him) used to accept the testimony of the followers of each religion against those of their own religion, but he did not consider the testimony of a Jew against a Christian, or of a Christian against a Jew, permissible. However, he accepted the testimony of Muslims against everyone, because their testimony is valid against all.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20628