Finally I set apache2 to get a single certificate for all subdomains.

[...]

# Go ahead and accept connections for these vhosts
# from non-SNI clients
SSLStrictSNIVHostCheck off


# Apache setup which will listen for and accept SSL connections on port 443.
Listen 443
# Listen for virtual host requests on all IP addresses
NameVirtualHost *:443


# Because this virtual host is defined first, it will
# be used as the default if the hostname is not received
# in the SSL handshake, e.g. if the browser doesn't support
# SNI.
<VirtualHost *:443>
  ServerName domain.localhost
  DocumentRoot "/Users/<my_user_name>/Sites/domain/public"

  <Directory "/Users/<my_user_name>/Sites/domain/public">
    Order allow,deny
    Allow from all
  </Directory>

  # SSL Configuration
  SSLEngine on
  ...
</VirtualHost>


<VirtualHost *:443>
  ServerName subdomain1.domain.localhost
  DocumentRoot "/Users/<my_user_name>/Sites/subdomain1/public"

  <Directory "/Users/<my_user_name>/Sites/subdomain1/public">
    Order allow,deny
    Allow from all
  </Directory>

  # SSL Configuration
  SSLEngine on
  ...
</VirtualHost>


<VirtualHost *:443>
  ServerName subdomain2.domain.localhost
  DocumentRoot "/Users/<my_user_name>/Sites/subdomain2/public"

  <Directory "/Users/<my_user_name>/Sites/subdomain2/public">
    Order allow,deny
    Allow from all
  </Directory>

  # SSL Configuration
  SSLEngine on
  ...
</VirtualHost>

So, for example, I can correctly access

https://subdomain1.domain.localhost
https://subdomain2.domain.localhost
...

Now, anyway, I have problems on accessing

http://subdomain1.domain.localhost
http://subdomain2.domain.localhost
...

Since I use a Mac Os, on accessing the "http: version", I get a default page "Your website." (instead of a error). Why does it happen?