Dynamics

When you use dynamics on musescore, I noticed that they snap to the note in closest to the bar you're working with, which is nice. However, this leads me to wonder if I need to use more than one dynamic when composing for say, a piano? For instance, a forte on the bar with the treble clef and a forte on the bar with the bass clef. Or, do the dynamics work for both bars unless otherwise written?

Thanks in advance.