This is taken from the Bridgestone website. It's maybe the manufacturer covering their arses but.....

Q: I have a pair of BT-020's fitted, but the rear needs replacing. Can I fit the new BT-021 with the BT-020 front?
A: At present, the BT-020 and BT-021 have not been tested as a pair. So to say they will work together would be foolish as there is no data to suggest they will.

Below is the link to the site
http://www.battlax.co.uk/faq.asp