MCVE

from random import randint

# List of indices corresponding to streambits that will be changed
chosen_streambits = []

# Number of letters that will be replaced with a "T"
num_t = 5

# Sample stream
stream = [["A", "N"], ["S", "U"], ["E", "R"], ["V", "J"], ["A", "N"], ["S", "U"], ["E", "R"], ["V", "J"], ["A", "N"], ["S", "U"], ["E", "R"], ["V", "J"]]

# This process will be done num_t times
for _ in range(num_t):
    # Select a random streambit
    rand_streambit = randint(0, len(stream)-1)
    # List of ranges, each centered on a previously chosen "T"
    plus_or_minus_two_off = [range(x-2, x+3) for x in chosen_streambits]

    while True:
        # Check if rand_streambit is within the ranges centered around other "T"s
        if rand_streambit not in plus_or_minus_two_off:
            # If it's not, add it to chosen_streambits and break
            chosen_streambits.append(rand_streambit)
            break
        else:
            # If it is, resample and repeat the loop
            rand_streambit = randint(0, len(stream)-1)

# Go through the chosen streambits (num_t long) and assign a random letter in them to "T"
for chosen_one in chosen_streambits:
    rand_letter = randint(0, len(stream[chosen_one]) - 1)
    stream[chosen_one][rand_letter] = "T"

# We would expect to see stream again, but with T's randomly scattered
#   through the list and never within two streambits of each other
print(stream)