Audience using Visitor Profile type have an optiont o allow one to include only those visitors which are not other tests.
Alternately, you may create two mutually exclusive testing groups using Profile scripts and using those in Audience.
Profile script may be something like this.
if (!user.get('twoGroups')) {
var random_number=Math.floor(Math.random()*99);
if(random_number <= 49){return 'GroupA';}else{return 'GroupB';}
}