In the UK, Chartered and Incorporated Professional Status is awarded by an accredited institution licenced by a national body (some examples licenced by the Engineering Council). Student Membership, Associate Membership, Membership and Fellowship are available.
When people append their Professional Status to their CV or to business cards, e-mails etc. it seems usual to only see one of these Institutions mentioned. Stephen Hawking listed two (FRS, FRSA), but he was Stephen Hawking.
The background to the question : I had investigated membership of the Institution that accredited my degree, obtained a few decades ago, but had not done anything further about joining. Through my employment and a merger of a University working group with another Institution, I seem to have accidentally ended up with associate membership of that other Institution.
Would it be considered "bad form" to become a member of several? Might it also depend on the level of membership and on whether you were Chartered or Incorporated (or equivalent) by each?