Here you will read that the buffer is just the same for all of them and you need to keep the total amount of enzymes below 1 tenth of total final volume. So:
2uL of DNA + 1uL of enzyme I +1uL of enzyme II + buffer + water to = 20uL.
"FastDigest® enzymes allow simultaneous digestion of DNA with two or more enzymes in one digestion reaction.
• Use 1 μl of each enzyme and scale up the reaction conditions appropriately.
• The combined volume of all added enzymes should not exceed 1/10 of the total reaction volume.
• If enzymes require different incubation temperature, perform sequential DNA cleavage: complete the first digestion reaction at the lower temperature, add the second enzyme and increase the digestion temperature for the second enzyme cleavage".