How do you figure? Even assuming humans and our animals accounted for 100% of the planet's biomass, after a single halving that would be reduced to... wait for it... 50%.
No. Percentages do not work that way - it would be reduced to 100% in your example. Let's use some absolute numbers: assume there are 99 humans to every 1 non-domesticated animal. Humanity is then 99% of the population. Halve the human population, and say it's now 49 humans to 1 non-domesticated animal. Now humans are 98% of the population.
Real numbers are surprising on the actual proportions. I offer you this visual representation.