1. Yes. They are OK and are short for "When (they are) discussed separately/together".
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Mr WordyThe only reducible ones that I can think of at the moment involve a past participle in a passive or adjectival role, or involve a present participle. For example,
"(When (they are)) eaten quickly, they give you indigestion."
"(When (I was)) walking home, I saw a strange light."
I'm not 100% sure that this is complete though.
T