Can you say a language still has a noun class system if a noun can take multiple class indicators at the same time? Like something that is long and flat getting the class indicators for both the long-class and the flat-class? Or is it something else?
My answer is yes.
In French, the plural word gens ("people") in general" is unique. It is clearly feminine diachronically and still demands feminine agreement for immediately preceding adjectives where there is a phonological difference and for associated quantifiers belonging to the same word group preceding the word gens. Anything not immediately preceding gens or not grouped with other preceding words are treated as masculine in keeping with the general rule in French that words generally applying to mixed genders are masculine. The word is also treated uniformly as masculine when followed by the preposition de (of) and a name of a quality or profession. Accordingly, you have sentences where gens can require different genders, sometimes within the same sentence. The rules are so unusual and so complex that even good native-speaking writers sometimes screw them up. Here are some examples of "good" usage that I think most well educated speakers would spontaneously use:
Tous(m) les gens sont là (All the people are there)
Toutes(f) les bonnes (f) gens sont là (All the good people are there)
Toutes (f) les bonnes (f) gens sont heureux (m) (All the good people are ; happy
Intruits (m) par l'expérience, les vielles (f) gens sont soupçonneux (m). Instructed by experience, the old people are very suspicious.
Certains (m) gens d'affaires
Certain business people
Nevertheless, gender in French is generally quite regular. If I replaced "gens" ("people") with "personnes" ("persons") in all the above examples, all the related words would be uniformly feminine, with perhaps a big change in authenticity, but little or no real difference in meaning.
Because of these strange rules, you have to say that gens, uniquely among French nouns, is both grammatically feminine and masculine at the same time when used in the plural. (On the rare occasions it is used in the singular, I think it is uniformly feminine.) The rest of the language does have its occasional quirks, but generally has a clean distinction between genders.
In German, which has masculine, feminine, and neuter nouns, there are a few common nouns, such as das Mädchen (the girl) whose neuter gender goes against "natural" gender. In such cases, agreement is strict within the noun phrase and generally within the sentence, but long distance agreement between sentences usually follows the natural gender in informal speech, so that the pronoun used in a following sentence to refer to the word Mädchen would be a form of sie (she)(feminine), rather than a form of es (it)(neuter) in violation of the formal rules of gender agreement.
In standard Swahili, in areas whereas it has always been natively spoken, I think that noun classes generally follow strict rules of concord that are metaphorically motivated in complex historical ways; however, in urban areas where it is now natively spoken and as an L2 language, nouns generally require mixed concord according to rules of animacy that apply to some parts of speech and some word classes, but not to others. Here are adapted examples taken from Wikipedia and my perhaps faulty memory (I have put the class number after the relevant Swahili word and tried to hyphenated the obvious Swahili agreement morphemes and to provide a literal English translation to make the morphemes clearer):
wa-naume (cl2) wa-kubwa (cl2) wa-li-anguk(a) (cl2) (normal concord)
(plural human) man big, they in the past fall (The big men fell)
(ma-)(cl6) rafiki (cl9/10) z-angu (cl10) wa-kubwa (cl2) wa-li anguka(a) (cl2) (the "ma" is optional, with it you have a triply mixed concord)
(plural expanse/round swelling(?)) friend(s) (inanimate animal (?)) my big, they in the past fall (My big friends fell) (Rafiki is a loan word from Arabic, which might partially explain its strange classification and behavior)
vi-tabu (cl8) vi-kubwa (cl8) vy-angu (cl8) vi-li-anguk(a) (cl8) (normal concord)
(plural tool/dimunitive) book big my, they in the past fall (My big books fell)
vi-faru (c18) wa-kubwa (cl2) wa-li-anguk(a) (c12) (mixed animate and regular concord)
(plural tool/complex diminutive) rhinocerous big, they in the past fall (The big rhinoceroses fell)
m-penzi (cl1) w-angu (cl1) a-na-ishi (cl1) hapa (normal concord)
(singular human) lover my, he/she now lives here (My friend lives here)
ki-penzi (cl7) ch-angu (cl7) ki-na-ishi hapa (normal concord, despite animacy)
(singular tool/dimunitive) sweetheart my, he/she now lives here (My sweetheart lives here.
Because of the clash between concord based on class and concord sometimes based on animacy, you get a mixed system in this type of Swahili, where a singular word can require different concord with different parts of speech. In each case, the noun itself is said to belong to a single class based on what prefix or lack of prefix it takes in the singular and plural. Apart from this animacy anomaly, class prefixes are quite regular.