Vivado Error: [DRC MDRV-1] Multiple Driver Nets

I am running into an error on Vivado. I am trying to run implementation to program my Basys board, but I am running into the following error:

[DRC MDRV-1] Multiple Driver Nets: Net ScrlFSM/RLC2B/DER1/DFF_R1/DFF1/nextS[1] has multiple drivers: ScrlFSM/RLC2B/DER1/DFF_R1/DFF1/Q_i_3/O, and ScrlFSM/RLC2B/DER1/DFF_R1/DFF1/Q_i_2/O.

This is my VHDL for my top level:

architecture Structural of xxxxxxxxxxx is

    component WordTo4DigitDisplayDriver is
    port (
            WORD    : in  STD_LOGIC_VECTOR(15 downto 0);
            PULSE   : in  STD_LOGIC;
            CLK     : in  STD_LOGIC;
            SEGMENT : out STD_LOGIC_VECTOR(0 to 6);
   ANODE   : out STD_LOGIC_VECTOR(3 downto 0)
        );
    end component;

    component PulseGenerator_1ms is
        port (
            CLK    : in  STD_LOGIC;
            PULSE  : out STD_LOGIC
        );
    end component;
   
    signal pulse_1ms : STD_LOGIC;

    component ScrollFSM is
        port (
            L       : in  STD_LOGIC;
            R       : in  STD_LOGIC;
            CLK     : in  STD_LOGIC;
            RESET   : in  STD_LOGIC;
            DISPLAY : out STD_LOGIC_VECTOR(1 downto 0)
        );
    end component;
   
begin

    Wt4DDD: WordTo4DigitDisplayDriver
    port map (
        WORD    => SWITCH(15 downto 0),
        PULSE   => pulse_1ms,
        CLK     => CLK,
        SEGMENT => SEGMENT,
        ANODE   => ANODE
    );

    PulseGen: PulseGenerator_1ms
    port map (
        CLK   => CLK,
        PULSE => pulse_1ms
    );
   
    ScrlFSM: ScrollFSM
    port map (
        L       => BTNL,
        R       => BTNR,
        CLK     => CLK,
        RESET   => BTND,
        DISPLAY (1 downto 0) => LED(15 downto 14)
    );

end architecture;

My code for ScrollFSM:

architecture Structural of ScrollFSM is

    component Reg_LOAD_CLR_2bit is
        port (
            D    : in  STD_LOGIC_VECTOR(1 downto 0);
            CLK  : in  STD_LOGIC;
            LOAD : in  STD_LOGIC;
            CLR  : in  STD_LOGIC;
            Q    : out STD_LOGIC_VECTOR(1 downto 0)
        );
    end component;
   
    signal currentS : STD_LOGIC_VECTOR(1 downto 0);
    signal nextS    : STD_LOGIC_VECTOR(1 downto 0);
   
    alias NS1 : STD_LOGIC is nextS(1);
    alias NS0 : STD_LOGIC is nextS(0);
    alias S1  : STD_LOGIC is currentS(1);
    alias S0  : STD_LOGIC is currentS(0);



begin

    NS1 <= (not S1 and not S0 and L) or (S1 and S0 and L) or (not S1 and S0 and R) or (S1 and not L and not R) or (S1 and not S0 and not L and not R);
   
    NS1 <= (S0 and not L and not R) or (not S0 and L and not R) or (not S0 and not L and R);
   
    RLC2B: Reg_LOAD_CLR_2bit
    port map (
        D => nextS,
        CLK => CLK,
        LOAD => '1',
        CLR => RESET,
        Q => currentS
    );

    DISPLAY <= currentS;

My code for DFF1:

architecture Behavioral of DFF is

begin

    process (CLK)
    begin
        if rising_edge(CLK) then
            Q <= D;
        end if;
    end process;

end architecture;

I read through possible causes of this, and it seems like I am tying multiple outputs together. I cannot find any case of that in my top level, so I'm not sure what the issue is. I tried resolving any possible errors in both my top level and in the rest of my VHDL but am still getting the same error.

The multiple drivers are in the ScrollFSM module, here

    NS1 <= (not S1 and not S0 and L) or (S1 and S0 and L) or (not S1 and S0 and R) or (S1 and not L and not R) or (S1 and not S0 and not L and not R);
   
    NS1 <= (S0 and not L and not R) or (not S0 and L and not R) or (not S0 and not L and R);

These are both concurrent assignments; they are not executed sequentially.
Maybe one of them should be NS0 <= ...