Help in making the transition from a 1-legged jumper to a 2-legged jumper.

I am strictly a 1-legged jumper, and this is because I am not much of a weight lifter and I am very skinny. I can dunk easily off of 1 leg but I come nowhere near off of 2 legs. Here are some of my 1-legged dunks:
[url]http://www.youtube.com/watch?v=Aw8isXbqgX4[/url]

When it comes to 2-legged jumps, the highest I can get is high enough to get all my fingers over the rim and hang on it. I want to be a 2-legged jumper because that will improve my rebounding and 2-legged jumpers seem to have more control in the air when going for the lay-up. I have a weight room available to me if necessary, I just need to know what to do.

Get your squat up(free bar back squat). 2 legged jumping is more of a strength jump compared to a 1 legged jump. If you've never hit the weights then just do a basic 3x5 on the squat 3 times a week and try to add weight each workout. Are you looking to just lift for your lower body or do you want to lift for you whole body(I suggest this.)?

Mostly lower body, but I'd be willing to work out every other area too. Everyone tells me I need to bulk up.

I would say proceed with caution. I made the transition from one-legged to two-legged jumper back in high school because it was more fun to dunk with two hands off two feet because I could hang on the rim easier. In my case it began to effect my breakaway speed and finishing ability. Any time I'd get into the open floor I'd try to set my two-feet for a dunk and too often it led to being tracked down and being fouled (or I'd lose my footing after trying to set my feet too quickly).

It took a couple of years for me to wise up and reintroduce my one-foot jumping and it was at that point my in-game dunks began to rise exponentially. So, with that, I'd fully endorse introducing the two-legged jump but not at the expense of your one-legged leaping. Get the best of both worlds.

Unfortunately, I don't know much in the way of increasing two foot jumping. I am not a specialist. I have a pretty solid two-foot vert but I'm not sure if my workouts are even the most effective. I do a little bit of everything. I jump rope, I mountain bike, I do lunges, jump lunges, dot jumping, max reach jumping, box jumping and obviously, a ton of basketball. It all seems to add up for me.

Like everyone is saying hits the weights. Make sure you include squats, Romanian dead lifts and some type of clean exercise. I would also do some upper body exercises even if you just do some body weight exercises

To add onto what Rake said, yes be careful. the more bulk you have, the less your 1 foot jump comes into play. You ever see guys like K-Love, Randolph, or Gib Baby ever dunk off 1 foot? Those guys are fat yes, but you bulk up enough you will become something like them and your 1 foot jump will feel foreign. Mostly what you want to do is lower body weights and plyos and stay lean. Graducally add bulk over the years so your joints and tendons get used to the weight over time. I mean it really depends on what your plans are for basketball tho. You don't look tall enough to be anything but a PG or swingman and for those guys speed and 1 leg jump is better. For bigger guys having a great standing vert (Like Dwight) is way better. So if I was you... I would do some weights, but mostly focus on plyos and getting more reactive. If you just play at a local gym or something and just want to be better at 2 for personal gain, then yea lot of weights and plyos. Just make sure you do your plyos on grass or a soft surface.

It took ages for me to get my gamertag linked, because EA site bugged for me. I finally got it to work not so long ago, and am finally able to see screenshots I've taken. And those were taken by accident, when I didn't know how to take 'em (only noticed the feature when "achievement unlocked" popped on my screen, for unlocking the one that you get from taking 3 screenshots). So they are mostly just closeups of ground, with couple knifekills
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I used to be the same way. Had incredible 1 legged jumping ability and could barely barely tip dunk off 2 feet. After wearing down my left knee and getting tendinitis in it, I decided to switch to jumping off of 2 feet. I started to stretch more, hop on the treadbike more, and just kept jumping off my 2 feet for everything. Now I can smash off of 2 feet easily. Bottom line is force yourself to jump off of 2 feet for a while and eventually you will get up there and it will become second nature. Make sure to keep your tendons and joints loose too.