Can coffee and tea be mixed together? Sure, it's called Hong Kong yuenyeung.
Yuenyeung is a coffee-milk tea blend traditionally made by mixing sweet milky tea with brewed coffee, although instant coffee sometimes stands in for the brewed version. It's made of a mixture of three parts of coffee and seven parts of Hong Kong-style milk tea, it can be served hot or cold.
Since yuenyeung contains caffeine which is not healthy for children, there is a caffeine-free variant called Children yuenyeung. It's made of Horlicks and Ovaltine, both are commonly available in Hong Kong.