This is a follow-up to my comment on your question with some references:
When using Configuration Split to distinctively use a certain conditional configuration on your environments, then you won't be able to access both, your development and production values at the same time (without reading in these splitted configurations manually).
The reason is, that configuration split hooks into the import/export process of your configuration. Once you import your configuration, the values for the respective environment will replace your database-stored configuration.
If you need to access both values within the same environment, you should change from config split to configuration overrides.
By keeping the production API value database stored across all your environments (in your configuration files that apply to all environments) and having your test API value defined as override within your e.g.
settings.local.php file, you'd be able to access both values using Config::get and Config::getOriginal within your dev/staging environments, while only the database stored value will be available in your production site.