Ok when you say midbass driver what do you mean. Just because it is a 6.5 or even a 8 does not mean it's just a midbass. When I hear midbass I think of the driver that works best from around 50-400hz. Most 6.5 drivers to come with ready made comp sets can be used for midbass/midrange duties. Some better then others, but never the less they can be used.
If I planning on building a 2 way active system i always look for a driver to has clean response for around 80hz up to at least 3.5khz. Now it no easy to fine, but not hard. At this point it makes tweeter choice a lot easier. I just do not like crossing tweeters any lower than this. I just thins the sound IMHO. If could I would go below 4.5-5khz with a tweeter
Now if you are planning a 3 way system midbass driver choice becomes easier. Now you look for a driver with good response from as low as possible to around 400hz. Any good midrange driver will be able to play from that point. As for the midrange to tweeter has rule of thumb as above.