@BenTurnbull - Glad to hear you have both assets playing back simultaneously. It sounds like the hardware is struggling render the multiple clips as they are currently configured. You can confirm this by adding a debugger / FPS counter to your project and turning things on and off to see where the bottlenecks are - We often use Graphy.
I assume you are building an APK to run directly on the headset, and not using Link / AirLink - Can you confirm? The standard Depthkit Studio renderer is technically compatible with on-device rendering, but works much better on desktop-class graphics hardware like that used when using the Quest in Link / AirLink mode. To address performance issues caused my the hardware constraints of rendering on the Quest itself, we developed the Depthkit Studio LIte renderer which sacrifices some visual fidelity for dramatically increased performance. Both Studio and Studio Lite renderers can be used in the same project, so you can even mix and match per object based on your aesthetic and performance needs.
For any objects which keep the Depthkit Studio renderer, lower the Volume Density to a value of 60 or less - This setting requires exponentially more computing power as it increases, so it’s best to keep it low when rendering on mobile hardware.
Also be aware of this recently discovered Studio Lite renderer bug and workaround if you go that route.