Using / in the reset= part of a castsequence DOES work. My hunter dps macro consists of reset=combat/target, and works just fine. Do a search for castsequence macros and you'll find many macros that use the / to separate the resets. Check out the macro guide at WoWwiki.
I'm not sure if using a comma to separate them does work. It might, I've just never come across it.

The reason your macro isn't working as you expect it to is because of how FTL works. The 'targetless' part of FTL is done by reseting your target each and every time you press your dps key. Here is the what happens on your slave each key press of a standard FTL macro (as your lightning bolt is):

1. /target [mod:alt.... your slave targets the toon you're using.
2. /cast [target=targettarget...... your slave fires a spell at the mob without actually targeting it.
3. /targetlasttarget. your slave goes back to their previous target, or clears target if nothing was targetted before.

So everytime you hit your dps key your slave switches targets (twice in fact, once to your current main, and once back to whatever it had before). This makes any castsequence with a reset=target in it reset.

I have not yet figured out a way around this, while keeping the targetless part of FTL intact.

What I currently do for those toons that need a reset=target castsequence is forgo the targetless part:

Change your big /target line to /assist.
Remove the [target=targettarget] from your cast line and leave it as a regular cast(sequence).
Remove the /targetlasttarget line.

Your slave will always keep their target, until your current main switches.