If they are 2 languages they shouldn't share a wiki. It could be heavily interlinked but duplicating an article into the British English wiki doesn't seem like a problem (all language wikipedias do it)
Other language wikipedias also value topics differently. A British English wikipedia would prefer citations from Britain. In many national cases it would merit having more than one article or articles of different opinion.
silly though ! and which wikipedia would be in English while the other pages are in American English (or in British English). I mean, neither side would accept their language name to be prefixed ; they are already infighting about what's the correct way to speak English.
Other language wikipedias also value topics differently. A British English wikipedia would prefer citations from Britain. In many national cases it would merit having more than one article or articles of different opinion.