There may be some misleading and some incorrect responses about the mod possibilities here.

Your question is what second mod to use on each - the game has fairly limited options to choose from.
As you seem to know, a high set is a wand and an offhand, each with a 15/-1 inscription.
Wand:
Inscription - "Seize the Day" which does 15/-1 to "spellcasting weapons" (wands or staffs)
Wand Wrapping - Either 10% HSR all spells or 20% HSR on spells of the wand's attribute.
(There are only two wand wrappings possible in the game. There are no wand wrappings that do anything else. Some folks use single handed martial weapons to get other mods like hp, enchanting and armor, but there is no way to get 15/-1 on a martial weapon.)
Wiki link to wand wrapping list -
http://wiki.guildwars.com/wiki/Wand_wrapping
Focus:
Inscription - "Live For Today" which does 15/-1 to offhands
Focus Core - There are only six available focus cores. Pick the option that suits your needs best: +hp
(+30 always, +45 in stance or while enchanted, +60 while hexed;) HCT of spells
(10% of all spells or 20% of spells from offhand's attribute.)
Wiki link to focus core list -
http://wiki.guildwars.com/wiki/Focus_core
You could check the options available from the various greens, weaponsmiths and collectors. You might find they provide an an easier and/or cheaper alternative to building the set out of inscribable gear.
(I don't recommend trying to build it out of oldschool/non-inscribable gold items - that'd take a lot more time and money to find just the right dual mods on items.)
Wiki list of greens -
http://wiki.guildwars.com/wiki/Unique_item
Wiki list of collector items -
http://wiki.guildwars.com/wiki/Collector_weapon
Wiki list of weaponsmiths -
http://wiki.guildwars.com/wiki/Weaponsmith
Personally, I only ever use high sets as a last-ditch-type effort. I tend to keep the 10% HSR all spells on my casters' high set wand and +30 on the high set focus. I figure if I'm needing to use afterburners in the first place, I may very well already be low on health.
The choice depends on when/how you're planning to use the set.
Cheers & GL!
Luny