Please refer to JohnKorchok's suggestion, that use a macro to achieve your needs.
But tag "office-word-itpro" focus more on general issue of Word client, if you have further issues of macro code, it's better to add a tag "office-vba-dev".
Thanks for your understanding.
If you don't mind, you can try to insert the document B as an object into document A and create a link between this object and document B.
Then you could edit the object in document A, the data would be saved on document B.
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