Is cinder access to the OpenGL extension NVX_linked_gpu_multicast？multi-GPU rendering
It’s a bit tricky to know exactly what you’re asking, as Cinder doesn’t prohibit access to anything, i.e. if you need very specific extensions you can active those as needed. Whether or not the framework itself supports particular extensions isn’t easy for me to say. But there’s a fairly easy way to find out - Download the entire source and search the source for the extension you mention.
If it doesn’t appear anywhere, then it’s safe to say that Cinder doesn’t natively support it, and if it needs heavy integration with other existing OpenGL functionality it may be too time consuming for you to bother with Cinder.