Is it a supported configuration to use 2 NSX bridges, 1 bridge from each NSX environment to bridge the environments to a VLAN with the gateway for the VLAN on a 3rd party router?
VMware NSX
VMware by Broadcom does not have specific design documentation for this type of coniguration. This kind of architectural design requirement would be better suited for a Professional Services Organization (PSO) engagement. PSO would be capable of reviewing the overall architectural design of this environment, and the specific use case for this requirement. A PSO engagement may also find/recommend a more efficient method to resolve the current requirements.
Please also reference the NSX Design Guide for NSX Edge Bridge use cases
NSX Reference Design Guide 4.2
NSX Bridge Use Cases