It's not necessarily a problem to double up workouts. You will probably be less fatigued for your rugby practices if you limit your resistance training. Rugby is a leg-driven sport, so going a bit light or not doing legs at all on these double days might be a good idea. Getting in some cardio is actually beneficia, but it would be a good idea to stay away from High Intensity Interval Training on those days, as HIIT demands legitimate recovery time.
Above all, you need to pay acute attention to how your body responds. If you find yourself overly fatigued, then you might want to cut back or cut out the two-a-days altogether. It's likely though that you'll be able to find a workable balance.