Order confirmation xsl:sort

We are having problems with sorting our seats in the correct order without using sli number.
This is what we currently have:

<xsl:for-each select="//GetOrderDetailsInfoResults/SubLineItem[li_seq_no=$liNo]">
<xsl:sort select="seat_row "/>

<xsl:value-of select="seat_row"/>-<xsl:value-of select="seat_num"/><BR/>

 </xsl:for-each>

It partially works, however, only sorts by letter and ideally we need to sort by both. If I add another sort line <xsl:sort select="seat_num "/>  then tickets are all over the place and not corresponding to the correct line.

Ideally it should be like this (<xsl:sort select="seat_row/seat_num"/> ), unfortunately if I use this sorting it is not working.

Many thanks,
Dmitri