How to show a view inside another view on only first row?
I'm not sure if Views nodes split module will be useful for your situation as @DnBase mentioned.

However, because you're using Views Field View; you can follow the following steps to achieve what you're looking for:


Edit field view actual view (referenced view)
Add Global: Null from CONTEXTUAL FILTERS menu 





From the next dialog box that appear, select Hide view from WHEN THE FILTER VALUE IS NOT AVAILABLE fieldset.
Then from WHEN THE FILTER VALUE IS AVAILABLE OR A DEFAULT IS PROVIDED fieldset, check Specify validation criteria and select PHP Code from validator dropdown menu 
Then add the following code in PHP Validate code field
From Action to take if filter value does not validate dropdown menu select Hide view


Code: 

if($argument == 1) {
  return TRUE;
} else {
  return FALSE;
}





Now edit the main view
Add Global: View result counter from FIELDS menu, then from field configuration dialog box, check Exclude from display
Then edit Global: View field, from field configuration dialog box, add this replacement pattern [%counter] into Contextual filters field 




By applying these steps, you'll notice that view field will only appear on first item.