The first system templates I created are for keyboard. The cleffs and braces are included in the groups. Both staves use the same coordinate system (top line is y=0). Unlike a single staff page, a system is a group in itself with each staff being a group within the system. This makes a more complex template, but the parts are well marked. Keep them intact so navigating the file is easier. Below is a skeleton of a two staff system.
<g id="body" transform="translate(180,690) scale(1)">
<g><desc> system 1-1 </desc><!-- indent 216 -->system 1 of page 1
<use xlink:href="#sys2-1"/>
<g><desc> ........................... staff 1-1 </desc>staff 1 of system 1
<g transform="translate(336,0)"><desc> msr 1-1 </desc>measure 1 of staff 1
<!-- ................................. beat 1 -->
<!-- ................................. beat 2 -->
</g>
<g transform="translate(1281,0)"><desc> msr 2-1 </desc>
<use xlink:href="#sysbar"/>
<!-- ................................. beat 1 -->
<!-- ................................. beat 2 -->
</g>
</g><!-- end of staff -->
<g transform="translate(0,240)">
<desc> .............................. staff 2-1 </desc>staff 2 of system 1
<g transform="translate(348,0)"><desc> msr 1-2 </desc>measure 1 of staff 2; system 1
<!-- ................................. beat 1 -->
<!-- ................................. beat 2 -->
</g>
<g transform="translate(906,0)"><desc> msr 2-2 </desc>
<!-- ................................. beat 1 -->
<!-- ................................. beat 2 -->
</g>
</g><!-- end of staff -->
<use x="2184" y="0" xlink:href="#sysbar"/>
</g><!-- end of system -->
</g><!-- end of body -->