I think your code will work if you put results=asis
in the chunk options.
<table border = 1>
<tr>
<td>
```{r results='asis', echo=FALSE}
library(xtable)
data(tli)
print(xtable(tli[1:20, ]),type='html')
```
</td>
<td>
```{r results='asis', echo=FALSE}
library(xtable)
data(tli)
print(xtable(tli[1:20, ]),type='html',comment=FALSE)
```
</td>
</tr>
</table>