Yes, a pair of numbers can have fewer than two common factors. This only happens when the numbers are co-prime -- that is, when the numbers have only one common factor. In that case, the common factor is always 1.
All numbers have common multiples.
Even numbers.
20 and 40 have 4 and 5 as common factors.
Okay. Give us a pair of numbers.
The common factor is 1.
21 and 42
12 and 24
12 and 24
' 1 ' is their only common factor. They're both prime numbers.
20 and 40
Just 1.
1, 2 and 4