Bob and Rob debate a question of law and do not agree. There isn't any material dispute between them to go to court about. They just want to find out what the legal truth is / who is right and who is wrong.
They can ask the question on this site and get a convincing answer but, should any of them later act in reliance on that answer and end up in a legal trouble, the answerer won't be liable. They both want an answer they can reasonably trust, and random people on the Internet just fall short of this requirement — no matter how seemingly reputable they are. Bob and Rob could even be debating just any good question from this site which one reputable member answers "Yes", another "No" — for example to kill time in COVID-19 lockdown.
They decide to bet. They are happy to pay to have their dispute resolved by an authoritative source (e.g. a lawyer, or a court of law).
How can they do that?
If any of them hires a lawyer, that lawyer will be protecting the interests of their client and therefore could potentially be biased to produce resolution that favours their client (lets them win the bet).
Can they jointly hire a lawyer? Are there any other solutions?