How do I use multiple GY-521, I2C sensor with Arduino?
Let's try removing some XY-problem aspects first.

The basic goal is to find angles of 2-joint system. This can be done with GY-521 breakout boards, however there are many cons in this solution.

First, you cannot measure angle with gyroscope. For stationary system it can be done with accelerometers, but for mobile system you need a fusion of the gyro and accelerometer data.

Second, while MPU6050 has built-in DMP, making it work is a pain. Many hobbyists usually give up and implement fusion in software. This is not hard, but considering the amount of sensors it will be quite a draw on CPU and memory requirements. If you go this way, I would recommend getting breakout boards based on BNO055 sensor, which has nice fusion DMP that is much easier to use.

Third, unless you guarantee your model will be used on flat level surface only, you also would need additional MPU on the body itself, and then quite a lot of math to make sense of all of it.

All these troubles can be avoided by installing simple rotary pots in the joints and then reading them with ADC. If you have enough analog inputs on your Arduino this would be all you need. Otherwise you can multiplex feedback channels via MCU-controlled analog switch, like ADG731 to a single analog input.

Finally, if you planning on using your model on uneven or non-level surface, I'd recommend adding one MPU in the body and load sensors in the "feet". With some (much simple than above) math you should be able to control leg movements with enough precision to scale minor obstacles.