For example here is a snippet from moonlight sonata.
There are two open strings here with a pretty big gap in between them. How should this be played?
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If you are using a pick, you can use a hybrid picking technique (or sometimes called pick and fingers).
Hybrid picking is a guitar-playing technique that involves picking with a pick and one or more fingers alternately or simultaneously. Hybrid picking allows guitar players who use a pick (plectrum) to perform music which would normally require fingerstyle playing
The fact that the guitar has six strings each tuned a 4th (or 3rd) apart means that large gaps like this can be relatively painless.
In this particular instance, you can trust the tablature that is displayed below the notes: don't fret with your left hand at all, just pluck both strings simultaneously with two different fingers on your picking hand (the thumb can reasonably be used for the lower note). This is a piece of classical literature, so you can safely assume it is intended to be played on a nylon-string guitar without a pick.