The syntax for a cast sequence is as follows:
/castsequence [options] reset=condition1/... action1, action2, ...
wich means a macro like this
#showtooltip
/castsequence [known:6343,known:260643] reset=target Thunder Clap, Skullsplitter;[known:260643] reset=target Rend,Skullsplitter
should be detected correctly. The macro itself is correct as the game executes it like it should.
But Macro Toolkit does not detect it correct:
