Go to Astro.com
and input the birth datas. Click on one of the charts data. It will take you to a page with lots of options. Scroll down to the last option on the right hand side called extended charts. Make sure on person is listed in the "horiscope for" option and underneath it add the second person. Scroll down a bit to type of chart option. In the drop down section, select synastry or composite (mid point works best usually) You can select show chart at this point. Play with that page a bit to tweak more information, like to include asteroids and such.