Approval Workflow for Purchase Documents per Item

Dear all,

One of my customers has a particular approval process for purchase documents which I try to implement using the standard feature "workflow".
There are three scenarios:

1) below 10k
2) between 10k and 100k
3) above 100k

Each scenario requires the approval of two users belonging to different hierarchies.

1) scenario 1: one user sequence 1, one user sequence 2
2) scenario 2: one user sequence 1, one user sequence 2
3) scenario 3: one user sequence 1, one user sequence 2

I created three work flows for each scenario with the correct filters (document type, amount, status::open) and three "Workflow User Groups" containing:

Workflow User Group 1
- User 1 Sequence 1
- User 2 Sequence 1
- User 3 Sequence 2
- User 4 Sequence 2


Workflow User Group 1
- User 5 Sequence 1
- User 6 Sequence 1
- User 7 Sequence 2
- User 8 Sequence 2


Workflow User Group 1
- User 9 Sequence 1
- User 10 Sequence 1
- User 11 Sequence 2
- User 12 Sequence 2

My work flow work fine: I request approval. Each of the sequence 1 users sees the request. NAV waits for one sequence 1 and one sequence 2 user to approve before changing the status to "approved". This fullfills the requirement of 4 eyes check by hierarchy.

Now, the customer says that this is not sufficient.
He wants to have one sequence 1 and one sequence 2 user only.
They should be fix per item.

My idea was to set a default "Workflow User Group" on each item containing each time the two people per default to approve the transaction.

Is there any way to represent this in standard without hardcoding a workflow for each "Item No."?

Thanks

Best regards,
Sébastien