This combination does not allow 2 animal companions
It's unfortunate, but it's true. The hunter's class feature animal companion says
At 1st level, a hunter forms a bond with an animal companion. A hunter may begin play with any of the animals available to a druid. This animal is a loyal companion that accompanies the hunter on her adventures. This ability functions like the druid animal companion ability (which is part of the nature bond class feature). The hunter's effective druid level is equal to her hunter level. If a character receives an animal companion from more than one source, her effective druid levels stack for the purposes of determining the companion's statistics and abilities. A hunter may teach her companion hunter's tricks from the skirmisher ranger archetype instead of standard tricks.
Emphasis mine. Thus a sacred huntsmaster Inquisitor 4 with the animal domain has an animal companion as if he were a druid of (his inquisitor level) + (his inquisitor level −3), making for a superior animal companion (for his animal companion class feature he's an effective Druid 20 (the maximum) at Inquisitor 12), but such a character deserves a beefy animal companion, having devoted a lot of his resources to this class feature.
The approximate assumed wealth by level for a seventh level character is 23,500 gp, so you could afford such an item but it would account for half your wealth. Of course, specific campaigns vary, and if you have a party crafter you can halve the cost to you. And youe WBL about doubles every two levels so probably by level 9 it's more affordable, if it's a major part of your build.
No, eidolon levels do not stack in any way with animal companion levels.
An eidolon is not an animal companion so cannot benefit from Pack Flanking in any event.
However, I suspect you should probably be asking instead what your actual problem is - unless one or both of you have a lot of rogue levels, there's perhaps other better ways to reap a +2 to hit on your opponent, you have mentioned no real reason that you need flanking specifically. For flanking you could possibly use Gang Up instead, with another party member or, you know, a summon.
Best Answer
Give the animal companion a magic item granting the feat Pack Flanking
The challenging prerequisites for the teamwork feat Pack Flanking make the feat extremely difficult for most other classes besides the hunter (using the special ability hunter tactics) and the cavalier (using the special ability tactician) to use in conjunction with their animal companions.1 Magic items are the easiest way to circumvent the feat's overly restrictive prerequisites.
The character must still meet the prerequisites of the feat Pack Flanking in the first place, but these items mean the animal companion won't have to meet those same—and for the companion, likely impossible—prerequisites.
1 To have one's animal companion meet the feat's prerequisites, one could award the animal companion a wayfinder with the random resonant power of Combat Expertise (just pop in an ioun stone and roll the 21!) or maybe the 3rd-party weapon tooth of the narwhal (and never have inland adventures again!) then the animal companion could take the feat Animal Ally (convincing the GM that an animal with 4 Hit Dice is as if the creature possesses 4 levels, which it is) (note that there's probably a rule against nested companions somewhere), but, afterward, one must still somehow increase one's animal companion's Intelligence score to 13 (a difficult and expensive but not impossible task). This is not recommended. Just use a saddle or ring.