Ultrasound and microwave scanners are developed to scan the breast on the presence of cancer. Typically, these modalities are used independently. Here we investigate if we can achieve an improved resolution if we merge both modalities into a single device. In particular, we test if we can improve the resolution of the resulting images by employing a joint inversion method on both data sets as compared to doing two independent inversions leading to two independent images. For the joint inversion an error functional is employed that contains two terms; the first one represents the mismatch between the measured and modeled acoustic and electromagnetic wave fields whereas the second term links the two reconstructed contrast profiles with each other. Results show that joint Born inversion leads indeed to an improved reconstruction as compared to two independent Born inversions.