I just started training with a triathlon coach in South Korea. I'm 5' 11" (180cm) and weigh 213 lbs (97kg). My BMI is around 30, so he's trying to get my weight down so that running won't put so much stress on my knees.

He's got me doing circuit training followed immediately by a 40 minute stationary cycle. So my heart rate gets up to around 145 and stays there for about an hour.

I'm starting to lose some weight, but I'd like to put on a little muscle mass as well. My coach won't let me lift heavy enough weight to work to failure on my circuits, so I'm thinking about supplementing my morning workouts with an evening workout (at another gym) doing some powerlifting (deadlifts, squats, bench, curls, tricep dips) with high weight and low reps.

Is there any reason that this would be a bad idea? Circuit training Monday through Friday, with low weight and 30 reps, plus powerlifting on Tuesdays and Thursdays, just to add a little muscle?

I think it would be ok. There are lots of people that do cardio in the morning and then lift at night. It would be too much for me but maybe not for you. The only problem I see is iIt can be hard to lose fat and gain muscle at the same time. To lose fat you need a calorie deficit and to gain muscle you need a calorie surplus however begginers can usually accomplish doing both at the same time. I would just make sure you dont over train. Working out should be fun if it becomes a chore then you may want to scale back. You also want to make sure you are getting plenty of healthy foods and protein to repair your body.

Yeah, but my concern is that my cardio in the morning consists of weight training on isolation machines. Here is my circuit:

Shoulder press: 20kg x 30reps
Butterfly: 20kg x 30
Bench Press: 20kg x 30
Seated Row: 20kg x 30
Seated Pulldowns: 35kg x 30
Tricep Curls: 20kg x 30
Bicep Curls: 20kg x 30
Leg Extension: 40kg x 30
Prone Leg Curl: 45kg x 30
Seated Crunch (gripless) 45kg x 30

I do two of these circuits, then jump on a bike for 40 minutes. I just want to make sure that training a muscle for endurance and then 10 hours later training the same muscle for strength is actually going to meet my objectives of adding on a little mass. I'd like to double all the numbers on the arm exercises, and be able to do 40kg x 30 reps on most of them.

i dont think it would matter if you are using the same muscle groups b/c you arent going to failure. Do your muscles get sore the day after the circuit? I wouldnt be going heavy using muscle groups that are sore. Doing the circuit may interfere with your recovery some what and you may not maximize muscle gain. but I dont think it would be a real problem. I mean if you have to do the circuit 5 days a week you are going to have to lift weights on the same day or not lift at all. I wouldnt recomend leaving out the weight lifting even if its not under optimal conditions. As far as wanting to increase the weight on your circuit I would think eventually you would get to that point even if you arent lifting weights using low reps going to failure but I'm sure that doing so would help you get to that point sooner. Because you will gain more strenght quicker as long as you arent over training. Personally I alternate doing a circuit 3 days a week with lifting weights 3 days a week. I work out my whole body during the circuit and the next day I am not usually sore however if I am then I do not work the sore muscle. But that is pretty rare. Does that answer your question?

Yeah it does, but I'd like to also hear from others (especially a professional) out there if there are any reading this thread...

Thank you, though, it does make me less nervous about doing it. I just want to make sure I'm avoiding injury.