I have an answer for you! :)
Eles confundem suco e leite. (They confuse juice and milk).
Supposedly if the context is clear though you could say Eles misturam suco e leite and have it mean, mix up as in mistake (instead of giving a whirl in the blender aka o liquidificador)
And, just because I am here, it is sumo in EP PT (Europe, Asia, Africa), rather than suco as it is in Brazil. :)
This made me think of this old superstition we have in Brazil that if you drink a mix of mango juice (suco de manga) and milk (leite), you'd get sick. This is actually not at all true and, unless you're old or live on the coutryside, most of course don't believe it. This old wife's tale has its roots in the days when Brazil had slaves. The rich landowners could afford to have cattle and thus drink milk; slaves, being not very free of wealthy, as slaves tend to be, would just get a mango and drink its juice. Water wasn't much of an option as the world still had to realise that not dying was worth a few bucks worth of sanitation.