@schoen thank you for your answer.
On server A I used this command:
./certbot-auto certonly --standalone --email info@example.com --agree-tos --rsa-key-size 4096 -d example.com -d www.example.com -d blog.example.com
And on server B that command (here also runs a other full Domain, which fine):
./certbot-auto certonly --standalone --email info@example.com --agree-tos --rsa-key-size 4096 -d mydomain.tld -d www.mydomain.tld -d cloud.example.com
So both runs its own challenge.
Maybe I missed something.
Thank you for your help!
Edit: in /etc/letsencrypt/live/ is just mydomain.tld on Server B