Never ever run with mis-matched tyres on a motorcycle. two wheels are unstable at the best of times, the tread pattern make a huge difference to the feel of the bike and the characteristics change, even the same make but different model of tyre also affect the ridability. The bike will feel 'slippery' and the front wheel will not be in sync with the rear, I can't make it clearer, but if you ride with different tread you will know what I mean. I quess if you only ride on dirt then may not make that much difference, but on tarmac it's a definate no no.
When I buy new tyres I tend to buy two rears and one front, only because I know that the rears will wear out so much quicker and more than likely I may not be able to replace with a matching rear later.