Secondary recipient email body is blank when sent using "Find Contribution"
To add additional 'To' in email on Find Contribution action is not supported because the html is build using contribution id, Hence manually added email will not have contribution id selected from Find contribution result. It may be little tricky to change the code to support this. Below patch should work if your email body doesn't have contribution tokens.
diff --git a/CRM/Activity/BAO/Activity.php b/CRM/Activity/BAO/Activity.php
index 4fe6e48..ef36a12 100644
--- a/CRM/Activity/BAO/Activity.php
+++ b/CRM/Activity/BAO/Activity.php
@@ -1124,12 +1124,16 @@ class CRM_Activity_BAO_Activity extends CRM_Activity_DAO_Activity {
       $contactId = $values['contact_id'];
       $emailAddress = $values['email'];
 
-      if (!empty($contributionDetails)) {
+      if (!empty($contributionDetails[$contactId])) {
         $subject = $contributionDetails[$contactId]['subject'];
         $text = $contributionDetails[$contactId]['text'];
         $html = $contributionDetails[$contactId]['html'];
       }
-
+      else {
+        $subject = $subject;
+        $text = $text;
+        $html = $html;
+      }
       if (!empty($details) &amp;&amp; is_array($details[&quot;{$contactId}&quot;])) {
         // unset email from details since it always returns primary email address
         unset($details[&quot;{$contactId}&quot;]['email']);

Other alternate method is to use Advance Search, Display result as Contact, add contribution filter. Select Contact and choose email action.
HTH
Pradeep