Welcome to Yamaha Synth!

Do you know what's the use of the botton Part1-2 link?

When the [PART 1-2 LINK] button is lit, turning the knobs will affect both PARTS together. It is that simple. If you are playing a LAYER, and you turn the [PART 1-2 LINK] = ON, then turning the CUTOFF Knob will cause the FILTERs of both PARTS to change together. If the [PART 1-2] LINK] = OFF while playing the LAYER, only the PART with the blinking cursor in the screen will be affected.

Call up the first PERFORMANCE

CONCERT GRAND = PART 1
AMB PIZZA = PART 2

Press [PART 1-2 LINK] = ON
Turn Cutoff hear the filter control both the Piano and the Pizzacato strings PARTS together.

Turn [PART 1-2 LINK] = OFF
Turn the Cutoff Knob - the filter of the Piano is the only one affected.
Press the DOWN CURSOR arrow so it is next to AMB PIZZA
Turn the Cutoff knob, hear the filter control only the AMB PIZZA Voice in PART 2

and How should I do to add other effects in the knobs of Assign 1 y 2 ?

In order to program the Effects and Controller assignments, etc., you want to look into getting the Melas VOICE EDITOR. This Editor is made by John Melas, a third party programmer... the Editor lets you get to all of the VOICE mode parameters - even building VOICES from the Waveform up.

Please the this article for an idea about what can be done with the Editor:
MX Study Synthesizer Basics