It's the same in English. Would you say:

(a) Hugo and you, you're coming with me! -- Hugo et toi, vous m'accompagnez! (correct)
(b) Hugo and you, they are coming with me! -- Hugo et toi, ils m'accompagnent! (wrong)

Using second person plural, you are addressing a group of people. Third person plural would mean that you are talking about a group of people with someone separate from the group:

(c) Hugo et Marie, ils m'accompagnent. Toi, tu restes ici. -- Hugo and Marie, they are coming with me. You stay here. (correct)

Hi Patricia,

Hugo represents a grammatical third-person singular.

Toi represents a grammatical second-person singular. 

So together they need to be expressed as "vous" - (all of you). They cannot be represented by "Ils" because this is a group of grammatical third-person plural.

Hope this helps.

Jim