brane23,
I know a guy who is both a lawyer and a professional engineer. I think he works mostly in patent law, where, obviously, he has expertise.
I cannot see a lawyer/engineer being much use as an expert witness. An expert witness ought to have specialized training in his area of expertise, preferably at the graduate level and subject to a lot of peer review. A lawyer/engineer would be a "Jack of all trades, master of none."
Stepping away from the subject of engineering but continuing with expert witnesses, I suggest that you read up on some of the satanic ritual child abuse cases that took place in the eighties in nineties, mostly in the US. Millions and millions of dollars were spent on investigations and trials, a lot of innocent people were charged, convicted and imprisoned over crimes that were phyically impossible. All of this was driven by expert witnesses who were poorly trained, not subject to peer review, and sometimes, marginally sane. There is a reason for standards.
JHG