There is a category error here on multiple levels. 1+1=2 is unfalsifiable in multiple ways.
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The inference that 1+1 equals 2 arises within a particular mathematical system that hinges unjustifiable axioms, and may or may not correspond with the system we are modeling. Those axioms are formally unfalsifiable within the mathematical system.
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We can in fact identify several alternate experiments in which 1+1=1 (such as, adding two drops of water together produces drops), but these experiments do not falsify the mathematical fact that 1+1=2. Instead, they show that 1+1=2 is a poor model of the experimental system.
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Your experiment demonstrates my point in another way. In an experiment system well matched to 1+1=2, you fail to falsify the claim attempts to falsify the claim 1+1=2. In fact, we cannot conceive of an experimental system well matched to 1+1=2 in which the experiment would demonstrate anything other than 1+1=2. This is so obvious you did not even actually do the experiment, but just asserted (correctly) what its outcome would be. That means 1+1=2 is literally unfalsifiable by appropriate experiments. In fact it is unfalsifiable because it is true!
Yet is not a problem for scientific theories to rely on math, even though key components of math are intrinsically unfalsifiable.