I had an issue the other week where I wanted to do a transfer from a community account but I didn't have its active key set up on Keychain. I ended up transferring out from my own account as I didn't check the account details on Keychain. After all, why would you check it when you trust it 😉
It is up to the website you are using to enforce or not which username will make the transfer. Both are already possible in Keychain.
I was making the transfer in peakd. What I'm saying is I was in account Z on peakd, and when I hit transfer it took me to Keychain and did the transfer from account X when I hit ok. Granted I should have checked the account name first (but remember I'm tech stupid 😊) I thought it would have given me some sort of warning message to say I'm not authorising account Z to do the transfer.