The thing about losing weight for MMA is to go into the weigh in, which is preferably a day before weigh in, at a lighter weight than the weight group you are in or as close to under that weight then put back on as much weight as you can before the actual fight. many pro fighters are experts at cutting the weight then putting it back on in short time.
A 65kg fighter will weigh in lighter than that if he can then put on as much as he can after the weigh in so he'll be well over 65kg when he actually steps into the cage. I know people that can put a great deal of weight on after a weigh in.
There's walk around weight, weigh in weights and fighting weights, all different for the same person.
My Husband is studying kinesiology, and works at a sports clinic and has found that to drastically drop and gain weight in such a short amount of time leads to Heart, kidney, and liver failure in the long term. Not to mention the dehydration and muscle weakness that results in the short term for when one does fight.
It's best to stay as close to your weight class withing 5-10lbs throughout your career.
Maintaining weight takes much more discpline, and is so much more healthy for the fighter/athlete.
I know many sports practice drastic weight loss and gain, but I feel it's very detrimental to the body, metabolism, and mental focus of the athlete.
But, it's a great example on how simoltaneously crosstraining two arts can immediately conflict with the training of both arts, health of the practitioner, and cause "muscle memory confusion" when a practitioner is developing "muscle memory" by repetitious training of two or more drastically different styles.
He was drilling wrestling techniques and habits all day every day (5 days a week) and then coming to train with us two days a week trying to fight his own body's, now natural reflex to respond with wrestling technique, and fighting that reflex to spar with strictly striking. = Big Hesitation in sparring.
When the WC training fully sunk in (The best thing going for him was that he hadn't taken another striking art before WC) he used his wrestling technique less and less. This happened because he started to come to class 3-5 days a week.
Then, the kicker was, when we taught him the anti-grappling.
He kept getting into trouble with his coach in wrestling because he kept reverting to the anti-grappling techniques we taught him. He wasn't doing it on purpose to be a smart ***, it's just that he started training with us 5 days a week and that's what his body wanted to do.
It was easier and quicker technique, and more natural body movement than wrestling, and he was starting to prefer it. Mentally, and physically.
Moral is, when crosstraining the body will decide which art it likes best, and use it more. Possibly never using the other style in combat, ever.
Or the body will get confused in sparring and hesitate constantly (what I call muscle memory confusion.)
Or the body will use both, but not to the maximum effenciency and skill level that it could in either art.
Cross-training can be good, if you spend the time to focus a strong foundation and application of the first art to the point it's almost like second nature. Then, adding another art would be easier and more beneficial to the martial artist. IMHO, based on my observations teaching and training MA myself.