For a proper noun, in this case let's say Morty, would one replace the "-y" suffix when using the plural case with "-ies" or keep it as an unaltered "-ys"?
You would never use the "ies" rule with pronouns: use "-s", or "-es" if the word ends in an s.
Also, referring to "Mortys" suggests that the two people involved are a group of some kind: for example, two friends called Morty who often hang out together. For this reason, you would tend to not use this construction in any other context.
"Are the Mortys there yet?"
"Will the Anguses be at the wedding?"
Note that on the face of it this second example sounds like you are talking about the Angus family, not two people called Angus. This perception depends on the commonness of the name Angus as a surname. For example, "Will the Smiths be at the wedding?" sounds even more like you're talking about a family, since "Smith" as a first name is extremely rare, and it's common as a surname (in some countries).
To avoid this confusion when talking about two people whose first name is Morty, a common mechanic is to use the initial of their surnames, or perhaps a nickname, to differentiate them.
EDIT: I just noticed that this question is a semi-duplicate of this one
which points out that the "use -es" rule doesn't just apply to names ending in "s" but a variety of other endings which would be hard to say if you just put an "s" after them.