The easiest method would be to prioritize your accounts for control.
You'll only ever have 1 of the 2 toons from account 1 at once.
You'll only ever have 1 of the 2 toons from account 2 at once.

If you were to go with the target=targettarget assist method this could work quite well.
Basically your "Big Button" controls both Targeting and Follow.

Search for the "No Focus Manifesto".

The Big Button is something like this:
/target [target=ToonA, nodead, help, exists]; [target=ToonB, nodead, help, exists]; [target=ToonC, nodead, help, exists]; [target=ToonD, nodead, help, exists]; [target=ToonA, exists]
/Follow

Normally you're running five toons, and this will force all of the toons to target Toon A, if Toon A is present and alive. If not they all target Toon B, etc. The other toons are never forced to target Toon E, because if only toon E is alive, you're actively controling them already and if none of the toons are alive, then the last conditional has your slaves target the master toon to follow them back on the corpse run.

I'm not sure how many characters it would take, but if your primary control was always the toon on account A, then you could make your big button read... Account-A-Toon-A as the first toon in the big button and Account-A-Toon-B as the second toon. Now we move on to the second account, and have the third toon in the Big Button refer to the first toon on account two. The fourth condition refers to the 2nd toon on account two. If there's space in the macro, you'd like to include each toon on account 3 and then account 4 as well.

Targeting four toons easily fits in the normal Big Button, but I'm not sure if you'd have the characters to include eight targets - six seems quite likely to me.

By only including say six toons, we have Toon A or B on account 1, Toon C or D on account 2 and Toon E or F on account 3. This allows for any toon on your first account to lead every toon on the remaining four accounts. In the event that the toon on Account 1 is dead, then every toon would target either Toon C or Toon D, whichever you were playing. The drawback will be, having party members die on Accounts 1-3, with survivors on accounts 4 and 5, with the Big Button not doing much.

Anyway, once you have all the toons targeting your chosen leader, you need to make a few macro's for them. For spellcasters, the format is like this:

/cast [target=target, exists, harm] [target=targettarget, exists, harm] Lightning Bolt
/cast [target=targettarget, exists, help] [target=targettargettarget, exists, help] Lesser Healing Wave

A macro goes through each conditional until it finds one which reads as true.

The first macro will allow any toon to cast Lightning Bolt at a hostile target.
However, if they're targeting your lead toon (who is not hostile), then the LB goes to the target of your target.
The Big Button has you targeting a friendly on account 1. So almost always you cast at the target of your account 1 toon.
But if for example Toon 4 is your miner and all the other toons are out of range, or you're playing Toon 4 solo, the macro allows your spell to land on a hostile.

The second macro is for a healer type toon.
The first conditional is target=targettarget.
So the healer is targeting your lead toon, and the heal goes to whoever they target, should the lead toons target be friendly.
If this fails, then it goes to the target of the lead toon's target, which will almost always be your team's tank.

For a class like a Hunter (with Auto Shots) or a melee type, preface your macro with an assist.
If your active toon is the party leader, it could be "/assist [target=party1]".
Jamba has an option to auto promote the master of your team to party leader.
You'd want a macro which reads, "/Jamba setmeasmaster all" on a key which is not broadcast.
When you switch to a new lead (which only happens when the previous leader is dead), tap this key.

This is probably not exactly what you were looking for, but it will function just fine.
The healer (any healer on any of your accounts) heals the target of your leader's target.
The DPS target their attacks at the target of your lead toon.
It would run best if your primary toons were the tanks.
If you had a tank on account 1, and a different tank on account 4, prioritize in the big button which one leads.