It's natural that both Farke and Tuchel would favour a 'double 6' midfield given their time together at Dortmund. The 6 traditionally is a DM acting as a 'single pivot' between the defence and the advanced midfielders ahead of him as in a 4-1-4-1 formation. The weakness of this is it tends to fail when facing a heavy press, making it difficult to move the ball up the pitch. It's also vulnerable to opposition overloads when defending.
In Farkeball's 'double 6' the DM or number 6 (Skipp) is partnered by the number 8 (McLean) who becomes an acting number 6 as well. Both act as a team to move the ball forward to the attacking players and to negate opposition attacks after the ball is lost. This was a key factor in our defensive improvement, succeeding where previous versions like Leitner and Trybull had failed.
This is also what Sørensen means when he says DF wants him to change to play as a 'number 6'. Gilmour would be coming to us to develop his skills as one half of the 'double 6'. As DH suggests, this will be at the heart of our tactics. What's harder to predict is whether the 'double 6' will be in front of a back 4 or a back 3, which would allow the WBs more freedom to get forward to support the attacking players.