Several times now, a piece I'm playing has asked me to play B# and C or E# and F at the same time. I'm very confused at what's this supposed to mean? The key is G minor/B flat, or 2 flats. Can anyone explain?
Given that the key signature has B♭ and E♭, I highly suspect that the piece is asking you to play B natural and E natural. Musically, it would be highly unusual for a piece in a key signature with flats to ask for those notes to be sharped. Doubly so given that the C and F aren't sharped. In the circle of fifths, F and C are the first notes to become sharp, while E and B are the last to do so.
While it is likely a case of poor editing as Dom suggests, if it's from the Renaissance or Early Baroque, it could actually be a faithful copy of the manuscript. It took some time for accidental signs to become standardized to what they are today, and flat/sharp symbols could conceivably represent flat/not-flat (i.e. natural) versions of the note. In fact, the modern sharp and natural symbols both derived from the same square-shaped b symbol ("hard b"), as opposed to the rounder b symbol ("soft b") that became the modern flat sign.
That said, if the piece were legitimately asking you to play both a B♯ and a C natural (as extremely unlikely a situation as that would be), then they would simply be played as the same pitch.
It is not strange at all. According to the AB guide to music theory by eric taylor, B# and C are a type of unison called diminished second. I believe there can be many more situations like this. I believe if i were to pluck a C note and then pluck the B below that but on another string, but bend that B up to the pitch of C then it will be called a sharpened B because the pitch C was already there and the pitch B was there and moved to a pitch en-harmonically equivalent to C creating a unison of a diminished second with the notes B# and C. To test this yourself tune your high E string a semitone up to F and pluck it and pluck the E on the 6th fret of the 2nd string then bend it a semitone up to the same pitch as F, you will now have the notes E# and F as a type of unison called diminished second. It would be important to note a more accurate diminished second is by saying an F note bending a semitone down to E while and E is already there will create a unison of E and Fb (F flat) called diminished 2nd because the minor second of E being F was lowered a semitone. This may not be done on an acoustic guitar without some contraption to bend 1 string down while another sustains without changing pitch so B# and C can pass as a unison called diminished second where you play enharmonic equivalents in a rare case. I would have to see the music sheet to tell you for sure whats happening in your situation but usually these enharmonic equivalents mean for you to play unisons on 2 separate strings.