How would we do this if we're to believe this is possible anyways?
The relevant rules say
Manifest Echo
you can use a bonus action to magically manifest an echo of yourself in an unoccupied space you can see within 15 feet of you
So if planes of existence are separated physically by an infinite amount of space, that would mean manifesting an echo in the space of a plane that you could see wouldn't be possible since the distance exceeds the limit.
That would rule out effects that allow you to see the material plane from the border ethereal, such as a potion of etherealness or the spell itself.
What about teleporting back into the plane you travelled from?
The relevant rules say
Manifest Echo
• As a bonus action, you can teleport, magically swapping places with your echo at a cost of 15 feet of your movement, regardless of the distance between the two of you.
In this case distance wouldn't matter, as long as you have that bonus action and the 15ft of move speed.
So say you're in a situation where you want to apply this. You're about to fight the BBEG and you've come prepared with two bags of holding. Your Echo is manifested already so it's your turn and you get within 10ft of them, using a free action to create the gate to the astral plane with 15ft of movement to spare. You're transported there but using your bonus action, swap places with your echo who's still in the plane you came from
Are there rules that don't allow this?
Edit: I've been informed manifesting an echo from the border ethereal has a much more general rule that explains why it is not possible, as stated in the DMG p.48
The Ethereal Plane is a misty, fog-bound dimension. Its “shores,” called the Border Ethereal, overlap the Material Plane and the Inner Planes, so that every location on those planes has a corresponding location on the Ethereal Plane.