You could setup a rule like this but it will not automatically override the existing UPS rule. You will need to modify that rule as well.
To make a rule like this you can set the condition to 'Subtotal for any column' then select the TotalWeight column and select is <= a value of 1.
Then for the calculation you would choose 'based on column subtotal' and choose the total weight column and 'times' then the factor you would like to multiply it by.
Once you do this you will want to update your UPS rule so that it has an additional trigger in the condition. In here you will set it to 'Subtotal for any column' then select the TotalWeight column and select is > a value of 1. This will ensure that the UPS rule will only trigger if the total weight of the cart is more than 1. With the rules setup like this it should use only one or the other, but not both.
