Do you not find a risk in the combination? Tank crash, infection, etc? That would be my only concern. Is there a way to control the salinity and temp on bothe tanks without the combined approach?
There's definitely risks. But we've just had better luck. My frag tank can be shut off from my main system if necessary and can run on it's own. We had red bugs a few years ago and had to separate the two. (What a disaster - don't trust a "friend" and skip QT'ing or dipping new corals)
Really not worried about a tank crashing to be honest. I know it sounds silly, but I'm just not concerned with it. We have so many controllers that alert us and someone is almost always home to monitor the tank.
Of course you can get the parameters fairly similar from one tank to the next, but they'll never be quite perfectly in sync when separate.
Our frag system also serves as a display refugium (55 gallon hooked to a 180). I think the addition of it also being the fuge is very helpful. The corals really do eat the pods in there.