You could always just add your words to the current dictionary by right-clicking the word when highlighted in the spell check and selecting ADD TO DICTIONARY. This way you won’t have to maintain or switch back and forth. Beside I think some other programs that you may be running use the same dictionary like Outlook.
This is not a good answer. The solution should apply to the billions of people using Word. Imagine billions of people all having to add words based on foolish word list deficiency. The original dictionary should have all 50,000 words. No excused or bandaids need apply!