Seems a little ambiguous. It's possible the Power Sword table provides some useful context. But without looking at Space Master, my inclination is to interpret it as both a Slash and an Electrical critical of the severity stated on the table. As Vector says, use the same roll for both criticals. (I don't see any mention of Heat criticals here.)
Sorry to piggyback onto the thread, but this just came up for our group: JDale, is the rule the same for RMU? Thus for example the Paladin has a spell that does an additional A Holy critical. Does RMU say that you use the same roll for both the normal critical and the additional A holy?
Yes.
"If multiple criticals are delivered against a target
for any reason, all criticals always use the same roll and
modifiers (so a flaming sword will cut and burn the
target’s arm, rather than cutting his arm and burning his
foot)." -- A&CL p.22 (PDF numbering) Beta 2 July
Extra criticals become OP pretty easily if they let you get multiple rolls for a kill result.
Power Sword table does up to J criticals, which are E Slash + E Heat. The description probably means to use Electrical crits in place of Heat crits for the high critical results. I'm not sure any real thought went into interpreting this for those who had to substitute the two-handed sword chart. Allowing dual crits in that case might be appropriate, as you still don't have a Power Sword (which crits against ATs 2, 5, and 6 starting at 19 and hits E crits against AT 1 at 82).