You're right about independence issue, especially if we assume BM is paying for her counsel. Here's an example of how the Supreme Court, listening to BigLaw, tries to enable ethical walls without expressly approving of them:
Rule 1.8. Conflict of Interest: Current Clients: Specific Rules
(f) A lawyer shall not accept compensation for representing a client from one other than the client unless:
(1) the client gives informed consent;
(2) there is no interference with the lawyer's independence of professional judgment or with the client-lawyer relationship; and
(3) information relating to representation of a client is protected as required by
Rule 1.6.
To me, this rule begs the question whether a lawyer can plausibly say s/he is able to ignore the influence or demands of the person paying the piper. In the BM/SD situation, where one marriage has partners in both firms, there is no argument that passes the giggle test IMO.
In fact, it occurs to me that BM's Rule 21 motion was intended to preclude the prosecution from discovering the marriage and demanding that E&N be disqualified by Murphy.
I take your "mere witness" point, which is well made, but I hesitate to embrace it before we know whether SD has actual exposure to charges as a conspirator or accessory. She has clearly been the focus of intense scrutiny, and her decision to help BM with package pickup seems to have opened up access to her accounts as the early search of her home did not. I don't know if they have enough to charge her but it could get very interesting for BM if she is subpoenaed and cross examined by the prosecution as a hostile witness and pleads the Fifth Amendment in response to questions along those likes.
Which brings me back to the question whether these two firms can serve these two clients with the degree of loyalty required by the Colorado Rules of Professional Responsibility.