LLM across different generation GPUs (RDNA3 and RDNA4)?

A user on the Level1Techs forum asks whether local large language models can be run across both an AMD Radeon RX 7900 XT and a Radeon Pro R9700, noting the R9700's extra 12GB VRAM would bring combined capacity to 42GB, and questions if mixing GPU generations (RDNA3 and RDNA4) would work. The post highlights a common concern among AI enthusiasts about multi-GPU LLM inference with heterogeneous hardware.