como hago esa union del più f con el ff

You are in the English forum so I'll answer in English (it's easier for me anyway).

Translation: How do you make the union that appears between the staves from più f going to ff

The best way in my opinion is to put an f under the start note, double click it and add the "più" to the dynamic and put the ff under the end note. You can then add a cresc. from the line that extends between the two dynamics. Select the cresc. and change both the Begin text and Continue text to be blank.