its somewhere in these forums.. kinda hard to search it though. but the basic idea is to make two /castsequence macros in the same macro, and add null returns for spells by using commas.

for example:
/castsequence [target=focustarget] lightning bolt, lightning bolt, chain lightning,
/castsequence [target=focus] , , , lesser healing wave

the commas with the blanks returns nothing in the second castsequence as does the space after the comman in the first castsequence.