I can tell you what I've done and can point out the restrictions.
The split wants to to be imagined as if the property was "two tenants" ( a pair of rental properties.) The Rules are going to work best on the same Hub as the devices. In a few cases, a specific device will need to be on the 'wrong' hub. Example: A stairway between floors with a motion sensor at the top and another at the bottom. The light is controlled by the motion. But a strict "split by floor" design would have one motion and the light on one Hub, with the other motion on the other Hub. This is an easy example of putting all three devices on a single hub and make the Rule a fraction clearer. (I want to be clear: it's not a 'sin' to leave them split across two hubs, but as you'll read later, it uncomplicates Events.)
Splitting also needs to accomplish a pseudo load balance. You've paid for the Hub, keep it busy. Or better said, try and keep them equally busy. It's the Radios that are the current limitation, try and be aware of the impact of having a lot of devices 4 hops away.
Many more "External Apps" than I initially predicted, work fine deployed on both 'active radio' hubs. It is clear that google, and alexa aren't happy to have 2+ hubs on a single account, but I expected the same with... Lutron, for example. Wrong Lutron not only works, it works as spectacularly on multiple hubs as it does on a single. Homebridge surprised me too, works on 2 hubs. (But I use it only from 'coordinator')
Amazon Echo Skill, Chromecast, Homebridge, Dashboard all live on my 'coordinator hub' along with NOAA Weather Alerts, Homebridge, ApiXU.
Link to Hub and it's mate Hub Link have two important restrictions. First, you would put Link to Hub on both of your 'active radio' Hubs and Hub Link on the 3rd (coordinator) and send events to the coordinator. You choose the "real" devices on the 'active radio' Hub(s) to send to the coordinator, But sending events
the other way: from the coordinator to the 'active radio' hubs is a problem. Link to Hub is a single instance and so it can only send to one Hub Link instance, not two. You will want to plan which 'active radio' hub needs the most traffic from the coordinator. Said another way... Link to Hub can be deployed a total of once per hub. It sends to one Hub Link, (which also can be deployed once per hub, but IT will listen to multiple hubs, which is how this works at all.)
The second restriction with Link to Hub is that it sends a limited set of Events/Attributes. Switches, Contact Sensors, Dimmers, Motion Sensors, all work great. But Locks and Garage Door Openers don't.
A Garage Door Opener, to take an easy to describe example, has two Attributes shown in the Device Info page:
Current States
contact : closed
door : closed
So it's a dual device, a Contact Sensor (think: tilt) and a door device (think: open/close), yet on the Coordinator, it will show as ONLY a Contact Sensor. You can display that it is open/closed but you can not make the Garage Door Opener operate. (Virtual switches and some RM works around it.)
Therefore, be prepared for workarounds to overcome the two Link to Hub restrictions.
[ giant long answer already... I'll stop now. ]