What is an efficient process for doing this? For each output from LLM1, you paste it into LLM2 and say "does this sound right?"?

If it's that simple, is there a third system that can coordinate these two (and let you choose which two/three/n you want to use?

Markdown files are everything. I use LLMs to create .md files to create and refine other .md files and then somewhere down the road I let another LLM write the code. It can also do fancy mermaid diagrams.

Have it create a .md and then run another one to check that .md for hallucinations.

You can use existing guardrails software to implement this efficiently:

NVIDIA NeMo offers a nice bundle of tools for this, among others an interface to Cleanlabs API to check for thruthfullness in RAG apps.