Thanks to Japanese twitters, I have found a potential answer on how Yoshiro descends alongside Soh.
In the Japanese plaque, there is no subject given.
Soh is the messenger of the mountain goddess and governs the world alongside Yoshiro. Descending into the world, guidance is provided by Yoshiro and the power of the masks is bestowed by Soh.
In ancient Japan, there's something called a Miko's Divine Descent. It's when the kami descends into her body. The kanji for descent used is the same one in the plaque. It's not the descending from heaven kanji. When translated twice, it calls it either the miko's descent or the god's descent.
When the plaque mentions this descent, could it be to reference this type of descent?
So when it's 'the two descended into the human world' it could really be that Soh, through Yoshiro, descends into the human world. This fits because why include Yoshiro in Soh's bio unless she plays an active role in what Soh can do?
Something else to wonder about: Soh's bio mentions her 'taking up her own sword to protect the maiden'. Why phrase it this way unless it's meaningful that Soh has joined the battle? I'd figure it was a normal part of the pact.
Possibly implying the defilement gets worse over time and eventually, Yoshiro needs help from Soh.