Python 3, 1 * (51 + 51) = 102

First function:

Output is a tuple of characters.

lambda a,b:1and sum(zip(a,b),())or(a+b)*(a-~b)//2+b 

Try it online!

Second function:

lambda a,b:0and sum(zip(a,b),())or(a+b)*(a-~b)//2+b 

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Python 3, 1 * (50 + 50) = 100

-2 bytes thanks to Neil!

First function:

Output is a tuple of characters.

lambda a,b:1and sum(zip(a,b),())or(a+b)*(a-~b)/2+b 

Try it online!

Second function:

lambda a,b:0and sum(zip(a,b),())or(a+b)*(a-~b)/2+b 

Try it online!