Dynamic Duo
I wanted to get Maevyn to 10 asap so she could get her pet. Few quests around so mostly grinding. I decided to run her with Sissle following, not grouped but with Sissle only helping in emergencies and doing a bit of healing. Maevyns first kill gave about 150 XP but after Sissle had healed her once or twice and given her Power Word: Fortitude her XP on each kill went down to around 75 and this was persistant through the remaining kills, even without Sissle's assistance. It's as if Maezyn was tainted by being helped: the assistance from a high level extends to being healed between fights, it doesn't just effect the XP for assistance during one fight. It might be that when that friendly passing priest gives you a buff they are actually crippling the rate at which you gain XP for half an hour. I will have to test this theory out properly. It will be nice to find a way to get back at people that wind me up: give them rank 1 Power Word: Fortitude (+3 stamina !!) and hope they don't dispel it by right clicking the icon.
Ultimately I realised that it was more productive XP wise to simply have both girls fighting ungrouped: Maez got about half the experience rather than the 25% she would get if grouped and the enemies died so much quicker it was overall better than levelling Maezyn solo. The only downside to not grouping was that Power Word: Shield can only be cast on group members.
Tried it on with a level 11 elite, using Maezyn to melee and Sissle healing but Sissle kept drawing agro and died on two attempts. She tried Fade but it was no use. In retrospect I think I should have had Maezyn fire arcane shot rather than just using auto-shot to try to pull it back. Hey, I'm learning group play here!
Got Sissle to 12 and Maezyn to 10. When the hunter gets the pet it is such a change in fortune, I was immediately back in the hunter groove, ploughing through beasts. No time to try running the three, Sissle had died outside Gnomeregan in a fight with three enemies and she was still on the Spirit Healer debuff. Fighting multiple adds gets very complicated very quickly and my brain seizes. I need to learn some set-piece moves for these situations.
I've been thinking about how having a healer around will improve my Hunter play. Apart from a bit more DPS and having Sissle heal the pet to save Maez's mana, how can I improve the fights where things go wrong? The main reason for Maevyn having to run away was when the pet drew too much agro and was going to die. It remains to be seen whether extra healing from Sissle will make the pet able to tank four or more monsters. However, I think it will be very difficult to control two characters running away. If Sissle ever draws agro I hate to think how I will handle it. Have her Fade, hope it works, then have Maezyn blast the mob is probably the best tactic. There's scope here for either brilliant play or spectacular wipes.
Interesting times.
I'm planning to go out and buy another copy of WoW and get Maezyn licenced properly so I can twink her with some cash. She's so poor that a rusty grey vendor trash gun she got dropped was an improvement to what she was carrying.
I'm hooked on this style of play now, I can't imagine having Maezyn running around alone. Can I resist running Maevyn and Pooky together? Probably not now I've realised I could do it. That will make Outlands less tedious. Oh no, that idea is too tempting..... respec Maevyn to Marksman for mega ranged dps and respec Pook protection as her pet 
Crusader Aura works with parties! AAAAARRRRGGGGHHHH stoppit brain. Chill, cool it, Calm blue ocean, Calm blue ocean, Calm blue ocean.... think of those interesting ruby projects.
).
Peter's Moblog
A few thoughts...
1. Your style of play (multi-threaded/asynchronous) - not sure if this will work, it's still theory at the moment, but would it be possible to produce add-ons for the sisters that could monitor whispers and respond accordingly? That way you would have more control over fights by one whispering the other to heal, shield, etc.
2. My style of play (single-threaded/synchronous) - boosted the hunter's engineering up to 300 last night (buying mats from the AH), making a set of jumper cables XL along the way. During the production of the cables, I needed several delicate copper wires. Aha! My shaman jewelcrafter can produce that but he needs copper bar for them. Aha again! My priest was grinding ore earlier and he has a nice stock of copper. So the priest sends the copper bar to the shaman, the shaman makes the copper wire, sends it to the hunter and hey presto, one shiny new jumper cables XL trinket :) I always thought there would be significant advantages to having access to several production professions but I never thought they would start to complement each other so well. Also later on I responded to another hunter looking for a goblin engineer - he also wanted the XL trinket. He had the mats so I said no charge when he asked but he insisted on a 10g tip. Anyways, when the trinket was handed over to him, he doubled the tip to 20g - not bad for the press of a button. I really don't like charging for producing something when the mats have been supplied (we're all in this together right, no cost to me, no charge to you?), but with tips like that the boosting cost will be covered fairly quickly.
UPDATE (just googled it): Forget #1, Blizz gimped add-ons so they can no longer target and cast spells. Bummer.