Pretty obvious, but double shield was not caused by having 2 tanks. Double shield was caused by Blizzard not patching the game after 2 shield heroes were made extremely strong. You can even look at the last year or 2 of Overwatch. Double shield wasn’t meta even though Sigma and Orisa were both in the game.
Literally what happened is they added Sigma who was able to do a bit of everything and also buffed Orisa again around the same time (Made her shield bigger in a prior patch and then buffed her fortify). Then they stopped updating the game for quite a while. There were small patches, but they never tried to make serious changes until echo was released nearly a year later.
I understand a lot of people were traumatized by double shield, but it was literally only meta for maybe a year and even then, it wasn’t a 2 tank issue. Blizzard just stopped updating the game.