routers.cgi*Graph[syn_1]: mygraph
routers.cgi*Graph[syn_ack]: mygraph
This will define the new userdefined graph, 'mygraph', which consists of 2 lines (the 'out' from each of the 2 Targets, since they both have noi set). You can add the options 'total' or 'average' if you want to have an additional Total or Average line (unlikely).
To stack the lines, then use:
routers.cgi*GraphStyle[mygraph]: stack
You can also use various other options, such as
routers.cgi*Title[mygraph]: Combined Graph
routers.cgi*ShortDesc[mygraph]: Combined
routers.cgi*Icon[mygraph]: interface2-sm.gif
and so on to customise this new graph.
For some examples, see the demo system at
http://www.steveshipway.org/cgi-bin/routers2.pl and look at the CPU graphs for the Linux servers, which use stacking userdefined graphs. You can see the CFG file contents by using the special link in the Targets menu.