Hi, @Stagiair
Your "Select 2" action should return an array which like this:
[
{
"Name": "xxx",
"ContentBytes" : "xxxx"
},
{
"Name": "xxx",
"ContentBytes" : "xxxx"
}
]
So you can refer to my sampl flow to combine the selection and the populate the word action.

And then in the send an email action, you can put this expression in the attachments property:

union(outputs('Compose'),body('Select'))
You need to ensure the action name if the same in your side , if not , you need to replace your action name to the red font text .
If this reply can help you , you can click mark this reply as solution (Accept solution) which can help more people, thanks in advance!
Best Regards,
Yueyun Zhang