You don’t need to buy anything.
To give a specific example
If you’ve got 50/34 chain rings
and a 10 speed cassette covering 11-28 which is 11-12-13-14-15-17-19-21-24-28
then your small/2nd smallest is 34:12 which is pretty much the same as 50:17, the large ring and 6th smallest cog
so there’s a big overlap.