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You can run two separate campaigns or adgroups even with the same keyword. Make sure you uncheck 'Automatically optimize ad serving for my ads' option, under the campaign settings. That way your ads will get displayed alternately.
One quick point of clarification about the 'Automatically optimize ad serving for my ads' option. This option really only affects multiple ads within the same Ad Group.
It won't have any impact on ads for the same keyword in different Ad Groups or campaigns.
Hope that make sense.