A lot of people do keep them together (We don't, more for practical reasons than anything else).
As I understand it, although chickens can carry blackhead they don't generally suffer symptoms, where as for Turkeys it can be fatal.
I think it is spread via faeces?? So 2 meter separation should be fine, I myself am not too worried about it with ours and just add apple cider vinegar to both turkey and chicken water anyway. I think there is something you can add to the feed too - cayenne maybe?