That doesn’t work because YAML allows you to specify keys of any type, so
[activerecord, activemodel]: 'test'
is a mapping with a single key, the sequence [activerecord, activemodel] whose value is 'test'.
Instead, you can use an anchor/alias:
activerecord: &my_value 'test'
activemodel: *my_value
However, there’s no way of attaching both keys to the single value in one key/value pair.