You need to created a custom field using a plugin (Jira Misc Custom Fields., or ScriptRunner).
You can then define the Groovy script in the scripted custom field to extract and sum the two other fields. You will need the IDs of the two custom fields to be summed, the example script below adds two custom fields with IDs 1234 and 4321:
fieldA = issue.get(customfield_1234)
fieldB = issue.get(customfield_4321)
return fieldA + fieldB
You can add extra code to check for null values when extracting the field value, e.g.
fieldA= issue.get(customfield_1234) != null ? issue.get(customfield_1234) : 0
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