I usually do my cardio first thing in the morning, around 9:00. Then at about 6:00/6:30 I head into the gym to work with weights. I feel it's best to space out the workouts as far as possible, so when I train for my second session my energy levels have recovered. I think it would be impossible for me to really hit the weights hard and heavy if I did cardio right before, same thing the other way around. If I was doing lets say HIIT cardio how would expect my body to work at 100% after?

HrdGain81 raises a very good point as well, if you workout with weights then plan on doing cardio it could be VERY catabolic, and I wouldn't really advise this unless you use some type of supplement in between (Amino Acids, whey, etc.)