Hooking up 2 batteries in parallel will NOT damage them, it will simply provide a longer battery life( 2x the mAH available, vs 2x the voltage in series). Some adjustable bench powersupplies can be damaged when put in parallel due to internal design, so read the instruction manual first before using them, it is simply a design issue, batteries cannot be harmed by being placed in parallel.
18V= new Warp board, guaranteed to fry it, 12V is Max it can handle
As to the Warp/E-Mag Bat/Revvy mod. The revvy will take 18V across the battery contacts no problem, as it has an internal 12V regulator(Main difference between it and a Shredder), the Warp needs a 9 or 12V reg though. simple way to do this is to go to Radio Scrap and get a LM7809 or LM7812 compatible part, and build the reference circuit that will be usually on the back of the box, with the 12V output being hooked up to the Warp. Note that this will hurt batterylife, as the reg will draw power as it uses heat to disapate the extraneous voltage. I'm going to design a better voltage reg for this mod, when I get the time, and my electronics gear down from my mothers, where it currently resides.
2k2 VF Cocker, STO/Eclipse Blade, Old-Style 14" Boomstick,
68AutoMag Classic Feed CF11023, Ring trigger.