Watermarking as a powerful technique for copyright protection, content verification, covert communication and so on, has been studied for years, and is drawing more and more attention recently. There are many situations in which embedding multiple watermarks in an image is desired. This paper proposes an effective approach to embed dual watermarks by extending the single watermarking algorithms in Xie and Shen (2005) [1] and Xie and Shen (2006) [2] for numerical and logo watermarking, respectively. Experimental results show that the resulting dual watermarking algorithms have a significantly higher PSNR than existing dual watermarking algorithms and also retain the same robustness as and higher sensitivity than the original single watermarking algorithms on which they are based.