I agree with Alan's advice. Just using your example of 'biscuit' vs 'cookie', the meanings are quite different depending on which side of the Atlantic you're on. So using both in the same text would most likely cause confusion. If you want to read more: http://blog.oxforddictionaries.com/2014/05/biscuit-vs-cookie/ :) It is probably best to stay consistent in your usage, whether that is spelling (the whole 'colour' vs 'color', 'realise' vs 'realize' thing) or vocabulary. With spelling it just looks better stylistically to be consistent. With vocabulary. it would be odd and potentially confusing if one moment you were speaking about the pavement, and then changed later in the text switched to sidewalk. Or if you were speaking about jam, and them switched to jelly (which means something different in the UK). Native speakers are familiar with the differences in terminology, and by staying consistent you are more likely to reduce the risk of misunderstandings happening.

I would recommend choosing your flavor (or flavour) of English and using that version in the same text. Since BrE and AmE words already have different meanings, you risk confusing your readers even more by using both words in the same text. Exceptions could be when the text is actually explaining the difference between the meanings, and also if you put the second word in parentheses, such as I did for 'flavor (flavour)', even though this is a spelling difference only.