You may be overthinking this. If the link benefits the end users of both sites, get the link.
However, you said yourself the sites are not targeting the same geo location which tells me the link is irrelevant. So you answered your own question, probably a waste of time to do it. However, one irrelevant link isnt going to cause a problem with any search engine.
If you succeed to let the lawyer write relevant content for the accountants' blog (something legal of interest specifically to accountants), and vice versa, it looks like a winner to me. Guest blog writing is all good as far as I know.