In modern mobile electronic systems, several high speed digital ICs often working simultaneously and can cause interference issues with nearby antennas at the same time. In this paper, a method to quantify the contribution of multiple noise sources radiating simultaneously is proposed. The reciprocity theorem based radio frequency interference (RFI) estimation can be decomposed into forward and reverse problems. Multiple imaginary Huygens' boxes are introduced to enclose different IC noise sources. In forward problem, the ICs are working and the tangential E and H fields on corresponding Huygens' boxes can be obtained. In reverse problem, the victim antenna is excited while the noise sources are turned off. On the same set of Huygens' boxes, the tangential electromagnetic fields from the radiating antenna can also be recorded. With tangential E and H fields in both the forward and reverse problems on different Huygens' boxes, the coupled voltage contributed by multiple noise sources can be calculated separately using reciprocity theorem. The RFI noise source quantification method is validated by simulation. A test structure with two simultaneously radiated traces and a planar inverted-F antenna (PIFA) is used.