Both Inputs Can Be Independantly Adjusted Using R1 For Input 1 And R6 For Input 2. R5 Controls The Fade. Q1 Provides Gain Of The Combined Signal.
 
 

            1       1     BT1 9V                                    8       2     R1,R6 500K
            2       2     C3,C1 47uF                              9       2     R9,R2 47K
            3       2     C2,C4 100uF                            10     1     R3 100K
            4       1     J1 INPUT 1                              11     1     R4 2.2K
            5       1     J2 INPUT 2                              12     1     R5 1M
            6       1     J3 OUTPUT                             13     1     R7 15K
            7       1     Q1 2N3563                               14     1     R8 1K