Many do but there are a few grafted varieties that will produce fine without a second tree present. However, as with all...they will probably produce more with a second tree. The two varieties must have roughly the same bloom times. There are a few nurseries out there that offer nice, grafted varieties...which is reccommended over the seedlings. Here is where I've ordered mine from and have not been disappointed. The plants were a very nice size and have grown well.
http://www.nolinnursery.com/