I'm currently using my Hubitat for 99% of my home automation needs. I still have a SmartThings Hub hooked up for NST Manager and Harmony Hub control. Yesterday I attempted to add an Aeon Gen 5 multi sensor to my hubitat. While I could add it to the hub just fine (by bringing it to within a few feet of the hub), once I moved it to its permanent location on the other side of the house it stopped responding. I tried running a z-wave repair command a few times, but it just didn't help. I'm guessing it's just too far away from the rest of my z-wave devices (though there are a couple that I thought would be close enough). Anyway, the room I have my SmartThings Hub connected is actually right next to the bathroom where I want the motion sensor. A workaround I am using is adding the Aeon sensor to the SmartThings z-wave network (it's the only device), and then using Hub Link to allow hubitat to respond to motion events from the sensor. My only concern is the possibility of some kind of interference caused by having two separate z-wave networks in the same house. I don't know enough about the technology to know if that's possible or not.
You can have primary and secondary z-wave networks. The use case you describe should not cause an issue in daily use, but might on occasion cause an issue when attempting to pair a new z-wave device with Hubitat. In that instance, just unplug the ST hub while attempting to pair with Hubitat.
While the above is true, I'm not really sure that's what the original poster is asking--sounds like they're wondering about two completely separate Z-Wave networks. And SmartThings doesn't officially support being a secondary controller anyway (not sure about Hubitat but also doubt it; looks like the above poster just popped off the Hubitat Z-Wave stick and used it with OZWCP on a temporary computer for the purpose of migrating the network; not sure how that would help here, but that's definitely interesting).
Back to the question: I don't know the technical details about Z-Wave, e.g., if divides its (various) frequencies into separate "channels" like ZigBee does or if all Z-Wave devices could overlap when communicating. If it does use smaller "channels," your two Z-Wave networks could be on separate channels, which should reduce or eliminate the possibility of them interfering with each other (can anyone actually tell me how Z-Wave works in this regard, by the way?).
However, on the practical side, I can say that I have two separate Z-Wave networks in my house, and they do both seem to work fine. Your issue could very well be what you suspect, a simple range problem that you solved with this repeater. I'd expect to have worse luck with ZigBee, which is not only crowded with existing uses like Wi-Fi (my own and all my neighbors') and Bluetooth and also generally has a bit less range, but even when I had three ZigBee networks (Hue, SmartThings, and Hubitat) going at the same time, I don't think there were any problems. I assume Z-Wave would be better, if anything. 
Following are the functions performed at z-wave MAC layer :
• unique network ID number (HomeID)
• up to 232 nodes in one network
• collision avoidance algorithm
• backoff algorithm
• automatic retransmission for reliable data transfer
• support for low-power operation via dedicated wakeup patterns.
Collision Avoidance and it's attendant backoff algorithm insure that a packet can make it through.
Note that a Controller is but ONE of the radios in a ZWave network. Every single ZWave device is competing for the same radio frequency. A controller does not take up more, it's functionally the same at the radio frequency level as every other ZWave device in you home, or your neighbor's.
Home IDs are but a single field in the packet sent via radio, therefore, multiple controllers or multiple HomeIDs (networks) are no more consumptive than adding another door sensor.
I doubt I explained this well, because there are still a lot of variables I tossed aside to simplify.
My opinion is.. yes, it's a range problem and simply swapping a dumb wall socket in the vicinity for a ZWave one, to act as a repeater, would solve it. Using a "spare" SmartThings hub pretty much acts as "proof" that it's a repeater issue. A $99 repeater vs a $35 wall socket repeater. 
Yeah, that "collision avoidance" is not what it sounds like: it only applies to messages that are all on the same network. It's not the kind of frequency hopping management that sophisticated zigbee coordinators do (but then zigbee in SmartThings doesn't do that either).
http://www.rfwireless-world.com/Tutorials/z-wave-MAC-layer.html
Anyway, back to the OP's question...
Z wave is intended to be a low-traffic low power network. Normally there's no problem having multiples in the same house. It's not really any different than an apartment building where everybody has their own hub. I've had as many as 5 separate zwave networks running at one time in my own house, which is under 2000 ft. although right now I'm down to two. No problems. I do keep the hubs a couple of rooms apart. But the other devices will just re-transmit if there's an occasional issue.
Where you can run into trouble is if you either do a lot of polling Or you run a lot of energy reporting devices. In either of these cases, your network traffic is probably hundreds of times higher than a more typical of zwave network, and consequently there is a greater chance of interference. It's basically like running a popcorn stand with A microwave operating almost constantly and then wondering why your Wi-Fi keeps dropping.
But if your zwave networks have normal zwave traffic patterns, and your hubs are at least 15 feet apart, it shouldn't be a problem. 
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