Python, swap variables every loop

You can use the modulous operator %:

for i in [0,1]:
    ...
    print('{} used {} words that {} did not use.'.format(a[i % 2], 50 , a[(i + 1) % 2]))

On the first iteration, i = 0:

i % 2       == 0 % 2 == 0
(i + 1) % 2 == 1 % 2 == 1

On the second iteration, i = 1:

i % 2       == 1 % 2 == 1
(i + 1) % 2 == 2 % 2 == 0

Note that the first i % 2 == i for this particular instance of your problem.

You can use the indice modulo 2 (%2):

a = ['first', 'second']

for idx in [0, 1]:
    print('{} used {} words that {} did not use.'.format(a[idx%2], 50 , a[(idx+1)%2]))

output:

first used 50 words that second did not use.
second used 50 words that first did not use.

Alternatively, if only two items:

it might be easier to read and maintain to do like this:

a = ['first', 'second']
x, y = a
print('{} used {} words that {} did not use.'.format(x, 50 , y))
print('{} used {} words that {} did not use.'.format(y, 50 , x))

If that is exactly what you're looking for, you could do:

for i in [0,1]:
    ...
    print('{} used {} words that {} did not use.'.format(a[i], 50 , a[(i+1)%2]))

This is my solution:

a = ['John','Doe']
amount = 50

# Use index to create strings to be formatted
s1 = '{0} used {2} words that {1} did not use.'
s2 = '{1} used {2} words that {0} did not use.'

print(s1.format(*a,amount))
print(s2.format(*a,amount))

Returns:

John used 50 words that Doe did not use.
Doe used 50 words that John did not use.

Or :

# Use index to create strings to be formatted
s = '''\
{0} used {2} words that {1} did not use.
{1} used {2} words that {0} did not use.'''

print(s.format(*a,amount))

Returns:

John used 50 words that Doe did not use.
Doe used 50 words that John did not use.