ruby0.49, 105 bytes

Result

AC

Duration

713ms

Code [DL]

print(t=gets())
while gets()
if k=$_.index("K")
print(" "*k+"a"*(t.index("T")-k+1))
else
print(t)
end
end

stdin

                                                 T
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
    K                                             

stdout

                                                 T
                                                 T
                                                 T
                                                 T
                                                 T
                                                 T
                                                 T
                                                 T
                                                 T
                                                 T
                                                 T
                                                 T
                                                 T
                                                 T
                                                 T
                                                 T
                                                 T
                                                 T
                                                 T
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

stderr

strace

10    execve("/bin/sh", ["/bin/sh", "/root/script", "/volume/CODE"], 0x7ffccbf6b140 /* 8 vars */) = 0
12    execve("/bin/ruby", ["/bin/ruby", "/volume/CODE"], 0x563a7535fc28 /* 8 vars */) = 0
11    execve("/bin/cat", ["cat", "-"], 0x563a7535fc10 /* 8 vars */) = 0
11    +++ exited with 0 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=11, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
12    +++ exited with 0 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=12, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
10    +++ exited with 0 +++