Overwatch 2

It is best to either duplicate heroes who will gain charge fast or a tank due to their high survivability, this is because you can be damaged out of your ult before you charge a duplicate ult reverting you back to standard echo. Two of the best I've found are moira, because of the self healing and the damage orb can hit multiple people and bypass barriers; and dva because she has both mech health and human health. Other than that when you are not ulting play carefully, quick play may work better as the enemy will be less coordinated but so will your team.

Edit: Forgot the most important thing, it has to be two different heroes.

Fun detail we found playing in a session tonight:

You can get several if not all of the character achievements while using Echo's ultimate. We had someone who needed to use Genji's ult but was using Echo. They copied the enemy's Genji, got the Quad-Kill and popped the "Slice and Dice" achievement. We also had the same with Roadhog's ult achievement to push 2 people off a ledge and die. They were Echo and had 'Hog's cheevo pop as well as this one.

Keep this in mind when you are boosting for this achievement as well as others! Could save a lot of time. Other achievements may work as well but ult kills definitely do work.

Sharing my personal experience just to help out with this one, I got it last night.

Ashe, D.Va, and Roadhog(generally, damage heroes) are all really good to clone to get the Ult form after Ulting. I personally did this with an Ashe and a Sigma.. I actually missed with the Y button and hit Sigma instead of Ashe again, but pushed a couple enemies and got the Ult at the last second by spamming Sigma's two attacks.

I did this on Assault Paris, the open lines with partial cover were great for Echo, and it was easy to stay alive and regen health as it took a long time while I was being careful! Be careful about getting in close range, I almost died once to a Brigitte but was saved by a teammate at the last second.

Make sure to spam the Y button if you are getting close and running out of time in the Ult form! I got Sigma's Ult as I was starting to switch back, I hit Y and turned back into Echo but it counted!!

D.Va's rockets can fully charge the Ult in one use I discovered, if you hit an enemy with them, I Ulted D.Va, then rocketed D.Va, got my Ult, and lost my mech before I could Ult as D.Va...
Roadhog Ult takes a lot of good hits, but if you get in close range with Roadhog, you'll get the Ult VERY FAST. Just, don't die after Ulting as Roadhog as I had happen almost every time.

Support characters are dicy to Ult--
Lucio 'can' be good, but most of the time I completely failed as Lucio.
Moira is a great support character to Ult IF you are close to the battle line.
I never ulted Mercy or the other supports... but if your team needs healing a support Ult will build VERY fast.

D.va is a great choice as you earn charge fast with defense matrix and her rockets. Lucio is is also a good choice as sound barrier can help keep you and your team alive in a fight. As staying alive just hang back and keep aware of your surroundings to not get flanked. I did this on defense on Rialto at the end of payload push.