I have a problem. I have a 4 day split with my workout regime a the moment, but I also have muay thai 3 times a week which are fairly intense cardio sessions.

Muay Thai starts at 6 and finishes at about 730 each night on Mon/Tues/Thurs. I don't have set days for my workout apart from I don't go to the gym on the weekend, that is my relax period.

My goals are to bulk up.

Now I usually try to get the gym out of the way during the day, so I can have a solid few hours break between my muay thai. Sometimes due to other commitments this is not possible, or sometimes I am just a lazy prick.
Sometimes I either
A) Go to the gym at about 3.30pm and finish at 5pm-ish, rest for an hour then go do muay thai.
or
B) Do Muay Thai at 6pm, finish at 730pm-ish, then head to the gym at about 9pm.

I always have a protein/carb shake between the two different training sessions, and if there are potatoes available I'll eat a few of them as well between sessions. If there's no good carbs available, I'll just have an energy snack pack (for you Australians, think Milo Energy Yogo type stuff).

But apart from the fact that having one after the other leaves me fatigued and not able to push to my natural max. Now I was wondering how having these two types of workouts (intense cardio vs weights) that close together? How should I plan it out properly, and more to the point, why? What is the body mechanism that tells me I should not keep these two so close? What are your suggestions (apart from telling me to cut out the cardio if I want to bulk up... not an option, I want to be fit and muay thai is important to me)?