Doing revision and homework (is/are) good for students.
There (was/were) once or twice that I failed my maths.
Which verb in the parentheses is correct?
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Beginning with your first question, according to https://www.writersdigest.com/online-editor/are-subjects-joined-by-and-singular-or-plural-grammar-rules, "Sentence subjects that have independent nouns connected by and are plural, thus requiring plural verbs (such as have)," which in this case is are. Yet your example begins with the singular gerund "doing," which according to several sources including https://forum.wordreference.com/threads/gerund-plural-nouns.2626241/ and https://magoosh.com/gmat/2012/gmat-grammar-gerunds-and-gerund-phrases/, appears to change the matter entirely. More specifically, "doing" here is the subject of the verb and thus requires its singular form (is). Regardless of the number of things being done, the sentence remains singular. Below is a bit of clarification.
Absent the gerund "doing," confirming which form of the verb to use in your example can be done by removing "and" and dividing the sentence into two. If the meaning remains the same, then the nouns are independent of one another and the sentence requires a plural verb. Thus, "... [R]evision ... is good for students"; and, "... [H]omework is good for students." "The meaning is the same and these nouns are thus independent of each other, making the original sentence a plural sentence and requiring a plural verb" (are), or "...[R]evision and homework are good for students."
When the subject of a sentence has multiple nouns connected by and that refer to a singular thing, then a singular verb is required. For example, Hot and bothered is his normal state of being. Hot and bothered is a singular phrase that is partly comprised of the conjunction and, so the singular verb (is) is the correct choice. Dividing the sentence yields Hot is his normal state of being; and, Bothered is his normal state of being. Here, the nouns are dependent upon one another, meaning that the sentence is singular and thus requires a singular verb (is).
Finally, "A gerund is the –ing form of a verb acting as a noun, in any of the 'noun roles' possible in a sentence. A gerund can be the subject, the direct object, or the object of a prepositional phrase." For example, in Changing colors, types, and styles is fun for me, changing functions as a noun (gerund) and is the subject. And in, People like my changing, changing functions as a noun (gerund) and is the object. However, in either I am changing; or, They are changing, changing is a verb and does not function as a noun (or gerund), so its accompanying verb must match the subject I (singular) or They (plural). Does that make sense? I obsessed about it for a while and I think it is correct, but I'm certainly open to anyone's input.
2) On your second sentence, according to https://www.grammar-monster.com/lessons/either_or_plural_or_singular_verb.htm, "When 'or' or 'nor' joins two things, use a singular verb if both things are singular. However, if one of the things is plural, use a plural verb." Thus, in your example "There (was/were) once or twice that I failed my maths", both once and twice are singular, so the singular verb was is the correct choice. Specifically, "There was once or twice that I failed my maths."
That said, I think that the sentence might sound better as some version of I failed my maths once or twice. I searched for a more definitive answer but found no source, automated correction or otherwise, that corrects anything except maths and still, it seems to be a questionable-sounding sentence to me. Obviously, the choice of maths is one of personal discretion, as is this suggestion, but it seems worth considering nonetheless. I'm sure that someone better versed in these matters can elucidate one way or another. I hope this is helpful nonetheless.