So I thought about synchronising different Arduinos to get more analog inputs.
Most beginners think that and they are wrong. The simplest solution is to use a multiplexer in front of the analogue inputs.
A 4051 chip will give you eight inputs on each chip and you can put one chip on each analogue input. So that is 16 * 8 = 128 inputs. If that is not enough their are techniques to get more.