Its not like the each tyre knows what the other tyre is doing, and for typical riding, so long as they are the correct size, I Dont think it would matter. Maybe in the past when tyre quality varied considerably it made a difference, but these days they are so similar, the average rider can't tell the difference. So unless you plan to be leaning the bike right over and getting your knee down on every bend, you should be OK. In fact I read somewhere that even if you replace one tyre with the identical model, the variations in age, and rubber compound from a different batch is the same as mixing tyres...