Is it grammatical to use ‘respective memorandum’ in court decisions?

THIS very interesting grammar question was e-mailed to me recently by a senior citizen and triathlon enthusiast who calls himself Tritorns:

“I am a practicing lawyer. I have read many court decisions that alternately use the word ‘memorandum’ or ‘memoranda’ in the following sentence: ‘All the parties are ordered to submit their respective memorandum or memoranda.’ My question is: Which is more grammatical, ‘memorandum’ or ‘memoranda’?”