We did that last year. Cristen swam all summer. She does a both a great bilateral front and back crawl. Roller skating is another one that she frequently enjoys while outside playing. Whethor or not a person is mixed dominant isn't the issue.
Many people are that perform way above average.
These things are determined in the womb and should not be tampered with. What you can encourage is good interhemispheric communication over the corpus collosum. You can find out which hemisphere is dominant for language by doing a dichotic listening test. The only sure fire way is to anethesize one hemiphere at a time and test for function. Obviously, you can't do that.