When done properly, stretches performed with the assistance of a partner can be more effective than stretches performed without a partner. This is especially true of isometric stretches (see section Isometric Stretching) and PNF stretches (see section PNF Stretching). The problem with using a partner, however, is that the partner does not feel what you feel, and thus cannot respond as quickly to any discomfort that might prompt you to immediately reduce the intensity (or some other aspect) of the stretch. This can greatly increase your risk of injury while performing a particular exercise.
If you do choose to stretch with a partner, make sure that it is someone you trust to pay close attention to you while you stretch, and to act appropriately when you signal that you are feeling pain or discomfort.