Combine parts?

It's possible, but you'll have to think through pretty carefully *how* you'd like the result to look. You'll have to think through which notes you want to end up in which voices, perhaps figure out which notes' durations can be altered to allow voices to be merged (since duration is not so meaningful for guitar), and for that matter, which notes you might have to sacrifice because guitar can't play as many notes or independent lines at once as piano can.

It's a complex enough proposition that I would personally work it out on pencil and paper first sitting down at the piano with a guitar on my lap, and then simply re-entering the music as it would need to be for guitar. I wouldn't start by blindly merging everything and then trying to fix the results - the fixing of the results will likely turn out to be more work than entering it anew.

But if you'd like to try, and your music is simple enough that its physically possible (eg, doesn't require more than 4 voices), you could use Edit / Voices to move the contents of the bottom staff from voices 1 & 2 to voices 3 & 4 (eg, exchange 1-1 then 2-4), then use the selection filter to exclude voices 1 & 2, copy the contents of the bottom staff, and paste to the treble staff.

If the music for piano is *really* simple - using only voice per staff - you could even just move the bottom staff to voice 2 rather than using voices 3 & 4.

If you have trouble conceptualizing any of this, feel free to attach the score you are trying to re-arrange. It's really much more of a musical problem than a technical one if you want to do a good job, though.

(Marc was typing at the same time I was and made some very good points. I'm working under the assumption that the two piano hands are simple and playable by a single guitar player with each line having been entered in 1 voice.)

Select the 2 parts in total. If this is the only instrument you can use ctrl-a

one the menu choose Edit -> Tools -> implode

You now have both lines in one. The only caveat is that the rhythm have to be compatible to combine them this way.

If you have different rhythms (1/2 note on one line and 2 1/4 notes on the other line for example) you will have to use voices. I would select all the right hand staff* then on the menu choose Edit -> Voices -> Exchange voice 1-2. Next press F6 to make the Selection filter show up. uncheck Voice 1. Select the entire right hand staff* and copy it (the rests for voice 1 will still be black if you did this correctly), I use ctrl-c for this. Click on the first beat of the left hand staff and paste what you just copied, I use ctrl-v for this. On the selection filter check voice 1 again and F6 will make it disappear.

*The easiest way to select one staff is to click the first beat of the staff and press ctrl-shift-End

You can now do what you need to put this on a guitar staff. Either delete the second staff (with the notes still there) and change the first staff to guitar or copy the first staff and paste it into an existing guitar staff.

Both Marc and Mike mentions "voices" above as the main tool to do this. Voices are described in the handbook here:
https://musescore.org/en/handbook/voices