Changing the instrument on one line

You'd need to play some dirty tricks:
Change Instrument to violin or trumpet
Use staff text and properties to switch between the 3 or 2 possible values, (normal, pizzicato, tremolo resp. normal, muted)
Finally in mixer assign different sounds

To be clear: MuseScore 1.3 supports some limited mid score changes - for trumpet and strings only, for changes from normal to muted (trumpet) or to pizzicato/tremolo (strings). This is done using staff texts as described. This works because there is a special configuration file instruments.xml that contains the details for every instrument, and it contains instructions for those instruments.

So you can either use those instruments then use the mixer to change the sounds into what you want, or you can edit instruments.xml on your system to add the necessary information for the instruments you wish to use.

It's also possible some of the scores you see that appear to be doing this aren't, and instead are simply having separate staves for the two instruments but are using Hide Empty Staves so you only see one at a time. That's another way to get this effect.

maybe you can take advantage of:
http://musescore.org/en/node/24765
add instrument, fill in only the staves where you want to put another sound, then "hide empty staves"

For MuseScore 1.3 more information, along with example scores, can be found here:

http://musescore.org/en/node/32246

Regards.