Split data into two training and one test sets

How can we split data int two training sets and one testing set using R?

I tried this, but I want to split train into two training sets:

set.seed(770)
sample <- sample.int(n = nrow(datazoo), size = floor(0.75*nrow(datazoo)), replace = F)
train <- datazoo[sample, ]
test  <- datazoo[-sample, ]

Thanks