With the v2.3 release you can now show two enharmonics together, like this:

This is done using custom note label text. Prior to v2.3 you could enter up to 3 characters as that's all that will generally fit by default. The limit has been increased from 3 to 7 characters.

Now the thing is, the font size of the note labels isn't calculated on the size of the text in the note (otherwise it could end up being lots of different font sizes across a single fretboard!) so if you enter lots of characters like the "F#/Gb" above, it won't initially fit inside the round note.

BUT we do have the ability to manually adjust the font size up or down. Here you can see I adjusted the note to be a bit bigger and reduced the font size:

There's a dev ticket to simply and automate this in future!