As Gefra says, you'll need to import the certificate from LB1 to LB2. If you're getting the error above, it is because the certificate you're importing doesn't match the private key that you have on LB2.
When you're in the Web GUI, make sure you select 'Archive' from the Import/Type drop down box. You can then upload the archive you downloaded from LB1.
To further check you can add the public and private keys to a temp folder such as /var/tmp/ and check the certificate modulus using OpenSSL:
If the modulus is the same, you have the correct key pair:
openssl x509 -noout -modulus -in public.crt | openssl md5
openssl rsa -noout -modulus -in private.key | openssl md5