`results = 'hide'` should try to collapse output, e.g., merge the source blocks below:

```{r, chunk-a, results = 'hide'}
nrow(iris)
ncol(iris)

iris
```

Keep the blank line:

```{r, chunk-a, results = 'hide', strip.white = FALSE}
```

Message blocks will be collapsed, too:

```{r, results = 'hide'}
nrow(iris)
message(ncol(iris))
iris
```