2 added 40 characters in body
source | link

With one Merge less, and using the operator form of Merge:

Merge[{association1, association2}, Merge[Association]]

(* <|
     "a" -> <|"b" -> <|"c" -> "txt1", "C" -> "TXT1"|>|>, 
     "d" -> <|"e" -> <|"f" -> "txt2", "F" -> "TXT2"|>|>
   |> *)

With one Merge less:

Merge[{association1, association2}, Merge[Association]]

(* <|
     "a" -> <|"b" -> <|"c" -> "txt1", "C" -> "TXT1"|>|>, 
     "d" -> <|"e" -> <|"f" -> "txt2", "F" -> "TXT2"|>|>
   |> *)

With one Merge less, and using the operator form of Merge:

Merge[{association1, association2}, Merge[Association]]

(* <|
     "a" -> <|"b" -> <|"c" -> "txt1", "C" -> "TXT1"|>|>, 
     "d" -> <|"e" -> <|"f" -> "txt2", "F" -> "TXT2"|>|>
   |> *)
1
source | link

With one Merge less:

Merge[{association1, association2}, Merge[Association]]

(* <|
     "a" -> <|"b" -> <|"c" -> "txt1", "C" -> "TXT1"|>|>, 
     "d" -> <|"e" -> <|"f" -> "txt2", "F" -> "TXT2"|>|>
   |> *)