If you use „Git for Windows“
>cd c:\Program Files\Git\etc\ssh\
add to ssh_config following:
AddKeysToAgent yes
IdentityFile ~/.ssh/id_rsa
IdentityFile ~/.ssh/id_rsa_test
ps. you need ssh version >= 7.2 (date of release 2016-02-28)
WICHTIG: Über Putty-Keygen muss der Private-Key über Conversions > Export OpenSSH Key (force new file format) exportiert werden