2 allers-retours. But grand-mère, not grande-mère.
Dictation exercise A1
The rules concerning the agreement of hyphenated words vary and are complex so I will just deal with the two examples you quote.
In the case of grand-mère the adjective grand will often not agree when followed by a feminine noun as in grand-route, grand-chose, grand-rue etc..
But the plural is accepted as in grands-mères.
in the case of allers-retours when it is two nouns, used in apposition, normally they will both agree -
des choux-fleurs, des bateaux-mouches, des chefs-lieux etc...
but there are lost of exceptions and multiple combinations with , verb +complement ( des couvre-lits) , invariable noun+ noun ( des non-lieux ) etc .when the agreement varies -
Probably best to learn each one as they occur...
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