Condition plugins: two on the same form?
You are responsible for only passing a sub-key of the form to each plugin configuration method.

Have a look at BlockBase::buildConfigurationForm() and the corresponding validate and submit methods. Note that this itself is already a partial form of BlockForm::form().

For a more flexible approach, have a look at https://drupal.org/project/page_manager, which has an UI that allows to add conditions and configure them in separate forms, but those forms also pass only a part of the form to those methods.

The main advantage of the page_manager approach is that BlockBase only works in a useful way with condition plugins that have a no-op mode, meaning, that you can configure them so they don't do anything (and that is the default configuration) because all condition configurations are saved and all plugins are invoked.

Yes, this is sometimes limiting and inconsistent, but that's how it works and I don't really have an idea to improve it...