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Experimental studies of single molecule mechanics require high force sensitivity and low drift, which can be achieved with optical tweezers. We built an optical tweezer setup for force measurements in a two bead assay. A cw infrared laser beam is split by polarization and focused by a high numerical aperture objective to create two traps. The same laser is used to form both traps and to measure the force by back focal plane interferometry. We show that although the two beams entering the microscope are designed to exhibit orthogonal polarization, interference and a significant parasitic force signal occur. Comparing the experimental results with a ray optics model, we show that the interference patterns are caused by the rotation of polarization on microscope lens surfaces and slides. The model qualitatively describes the pattern and the dependence of the parasitic force signal on the experimental parameters. We present two different approaches to experimentally reduce the crosstalk, namely, polarization rectification and frequency shifting.