Correct, the hue bridge forms its own mini network with any devices connected to it and so they don’t act as repeaters for the Zigbee network created by a smartthings/Aeotec hub.
With no repeaters, you have what is technically referred to as a “weak mesh.“ How many sensors do you have?
It would probably be a good idea to add a couple of Zigbee repeaters. You can get an IKEA tradfri plug-in pocket socket for about $10 if you buy it from IKEA directly, or a similar price in many other countries. These typically work well as repeaters if you have at least a V3 hub, although there are some reports that they may confuse IKEA buttons when used in a smartthings set up. I’ve used these in my own house and always been happy with them. But there are lots of other choices.
If you’re interested in learning more about building a strong mesh, see post 11 in the community FAQ on wireless range and repeaters. I will link directly to that post. You can click on the topic title to go to the full thread and after you re-post 11 go back up and read the whole thing.
I think you’ll find that most of us who are interested in network functions, and therefore are typically the people commenting in threads like this, try to have pretty strong networks with multiple repeaters. 
Anyway… Adding the second hub would at a minimum have increased some of the Zigbee network “noise“ in your house, which can cause individual devices to have a little more trouble connecting, and without a strong mesh, messages can then get lost. So this issue is definitely something that adding the second hub might have caused.
But you can certainly run multiple Zigbee networks in the same building, I’ve had as many as six or seven at the same time. (On different platforms: I’ve never run two smartthings hubs in the same building, although there are people who do.) It is a good idea to keep them off the same channel if you can and it’s always a good idea to have a strong mesh for each. That’s what protects you against occasional moderate interference.