Voice as an different instuments

As far as I know, this is not possible. You can make a score that looks like your example, but it won't playback that way.

I think this is a very standard piece of orchestral writing with two oboes on the upper staff and two bassoons on the lower staff. It can easily be notated (with the correct playback) byusing voices:
https://musescore.org/en/handbook/4/working-multiple-voices

"So is it possible to assign (make one voice as one instrument and the other one as something else) or merge 2 instument melodies into 1 stave as different voices?"

Sorry, I didn't actually answer your question.

AFAIK technically it is not yet possible in MS4.
It is partially possible in MS3, and I use it all the time for creating practice files for choirs. We often use closed stave SATB, so three voices remain as vocal, while the Sound for the fourth voice is set in the Mixer to a distinctive instrument such as flute (for Soprano), oboe (for Alto), or bassoon (for Tenor or Bass).

Example scores can be played here:
SIMPLE SCORE (closed stave SATB)
https://rootham.org/playlist/smb/canticles/nunc_dimittis_parish_psalter…
LARGER CHORAL WORK (open stave SATB)
https://www.beaminstersingers.org/practice05.html

How it is set up can be seen in the attached SATB closed-stave Alto practice file. In summary:
1. Use vocal options Women and Men as the instruments
2. Use the hidden SA and TB items from the Text palette (MS3 only, removed in MS4)
3. In the Mixer, expand the Women and Men to show the separate channels
4. In the Mixer, change the Sound for the specific voice to use a distinctive instrument
5. In the Mixer, leave the Sound for the three other voices as Choir Aahs Expr and reduce their volume

And this is why I still use MS 3.6.2 for all my serious work. ;-)