The first thing to understand about patents, especially in "greenfield" areas - no previously existing products, devices, methods, etc. - is you want to make the patent as broad as possible, without being so overly broad that it's unpatentable. You've identified two methods? Good -- write claims which teach both of those methods.
You'll notice I used the word "teach", because that's what you're creating, a "how to make or do one of these things". You want to teach as much as you can, with as much specificity as you can.
So, let's say you've invented "A method and device for peeling potatoes using recycled disposable razor blades", and you can rotate the potato or you can linearly move the potato. How do you rotate it? How do you control the rotation? How many different ways of rotating a potato next to a recycled disposable razor blade can you think of?
Once you've done that -- rotating and linearly moving the potato -- think about rotating the blades or linearly moving the blades. What about other forms of recycled blades? What about utility knife blades? Disposable plastic forks?
On to your actual questions. Do you need to separate patents? No. Patents cost money to file and prosecute and as long as all you've disclosed is "A method and device for peeling potatoes" you're fine. Does it matter if the potato peeler can be operated both manual and electrically? Sort of -- if you're just cranking a crank on your potato peeler, it would be "obvious" that hand-power could be replaced with an electric motor, but you should still claim that the potato peeler can be operated with a hand crank or an electric motor.
Remember that your goal is to teach someone as many different ways to create, operate, implement, embody, etc. the potato peeler. In the US Patent Office you get your first 20 "claims" for the same price. Use all 20.
Best of luck -- patents are amazing things to have on your resume.