R, knitr, xtable using both add.to.row and hline.after

Im trying to create a table with xtable

in R using knitr

. I have inserted a header and additional \hline

s. Im using the command add.to.row

to insert my title at the top and also halfway through the table. I can't seem to execute the command correctly hline.after

.

I would like my table to look like two tables (albeit one), with an hline after the 4th row of numbers and before the second header. After using the command, add.to.row

I cannot put lines in there.

Any suggestions?

\documentclass{article}
\usepackage[margin=.75in]{geometry}

\begin{document}

<<table, results='asis'>>=
require(knitr)
require(xtable)    

# Generate some data
set.seed(1683)
dat <- rnorm(48)
tab <- as.data.frame(matrix(dat, nrow=6, ncol=8))

addtorow <- list()
addtorow$pos <- list()
addtorow$pos[[1]] <- 0
addtorow$pos[[2]] <- 0
addtorow$pos[[3]] <- 4
addtorow$pos[[4]] <- 4
addtorow$command <- c(' & & \\multicolumn{2}{c}{Group 1} & \\multicolumn{2}{c}{Group 2} & 
                     \\multicolumn{2}{c}{Group 3}  \\\\\n',  
                   " name & $\\hat{\\beta}$ & A & B & A & B & A & B  \\\\\n", 
                   " & & & & & & & \\\\\n",
                   " name & $\\hat{\\sigma}$ & A & B & A & B & A & B  \\\\\n")


print(xtable(tab, align="rrccccccc"), add.to.row=addtorow, ,     include.rownames=FALSE, 
  include.colnames=FALSE, hline.after = c(-1, 0, 4, nrow(tab)))
@


\end{document}