Using two subordinating conjunctions in a single sentence is grammatically incorrect. Subordinating conjunctions are used to introduce dependent clauses that provide additional information about the main clause. They connect the dependent clause to the main clause, but they are not meant to be used in conjunction with each other. In your example sentence, "When talking something in general, does it need 'a' or 'the' even if it is a countable noun?", the subordinating conjunction "when" is used to introduce the dependent clause "talking something in general." Adding another subordinating conjunction such as "if" after "when" would create a grammatical error. To correct the sentence, you could use only one subordinating conjunction or rephrase the sentence to eliminate the need for two subordinating conjunctions. Here are a couple of possible alternatives: "When talking about something in general, does it need 'a' or 'the' even if it is a countable noun?" "If we talk about something in general, does it need 'a' or 'the' even if it is a countable noun?"

Using two subordinating conjunctions in a single sentence can result in awkward or incorrect syntax. Subordinating conjunctions are typically used to introduce dependent clauses that modify the main clause of a sentence. It is generally recommended to use one subordinating conjunction per dependent clause to maintain clarity and grammatical correctness. In the example sentence you provided, the subordinating conjunction "if" is correctly used to introduce the dependent clause "even if it is a countable noun." Adding an additional subordinating conjunction like "when" before "talking something in general" would not be grammatically appropriate. To revise the sentence while maintaining its meaning, you could use a coordinating conjunction like "or" to join the two phrases: "When talking about something in general, does it need 'a' or 'the,' even if it is a countable noun?" Alternatively, you could restructure the sentence to use two separate sentences or use a different subordinating conjunction if it aligns with your intended meaning: "When discussing something in general, does it require 'a' or 'the,' even if it is a countable noun?" In summary, it is generally not recommended to use two subordinating conjunctions in a single sentence. Instead, consider revising the sentence structure to maintain clarity and grammatical correctness.