Normally, with OpenSSH as SSH client and 1 SSH Key set under Options>General>SSH Client Configation, when SourceTree starts a terminal shell pops up asking the user to authenticate the key. Which is great.

If another key is added via Tools>Add SSH Key, this is appended in the SSH Key input. Each key is separated by ";"

What I would expect is that from shell I should be prompted in turn for each one of the keys.
What actually happens is that the shell opens and closes immediately, without prompting.
The same happens with 1 key and a trailing ";", or with a non-existent key path.

It would seem that ";" breaks the parsing.

Happy to provide more information if needed to investigate this.

Thank you,
vinz

Workaround

 Replace Semi-Colon - ( ; ) with two double quotation ( " " )

Example

C:\Users\<User>\.ssh\id_rsa1" "C:\Users\<User>\.ssh\id_rsa2