My api requires two fields in the header and it's described in the config/l5-swagger.php like this:
'securityDefinitions' => [
'securitySchemes' => [
'app_id' => [
'type' => 'apiKey',
'description' => 'API integration application identifier',
'name' => 'X-App-Id',
'in' => 'header',
],
'access_token' => [
'type' => 'apiKey',
'description' => 'API integration access token',
'name' => 'X-Access-Token',
'in' => 'header',
],
],
'security' => [
[
'app_id' => [],
'access_token' => [],
],
],
],
On the generated api documentation, when I click Authorize it renders the form with separate fields like this:
How do I tell l5-swagger to merge both of these fields together and not be treated separately?

Your code is correct. This is just how Swagger UI renders API key pairs. Here's an existing enhancement request to improve the representation of API key pairs:
https://github.com/swagger-api/swagger-ui/issues/3521