Tax type uniqueness

In Slovenia, we have separate VAT accounts for VAT-registered clents, and separate ones for the others (non-registered).

So, I have to set the following taxes:
20% VAT - for VAT-registered clients (account 26000)
20% VAT - for the others (account 26001)
8,5% VAT - for VAT-registered clients (account 26010)
8,5% VAT - for the others (account 26011)

On the other hand, for us (as a VAT-registered company), the account is generally (to make it simple; can be different, but depending on the origin...) always the same (16000 for 20% VAT / 16010 for 8,5% VAT).

As I was not able to enter the combinations:
20% VAT | 26000 | 16000
and
20% VAT | 26001 | 16000

I had to modify the checking function (so only the sales account has to be unique):

=======================================================

--- taxes/db/tax_types_db.inc.ORIGINAL    2010-12-10 14:12:45.000000000 +0100
+++ taxes/db/tax_types_db.inc        2010-12-22 09:47:30.000000000 +0100
@@ -102,8 +102,7 @@ function is_tax_gl_unique($gl_code, $gl_

    $sql = "SELECT count(*) FROM "
        .TB_PREF."tax_types   
-        WHERE (sales_gl_code=" .db_escape($gl_code)
-        ." OR purchasing_gl_code=" .db_escape($purch_code). ")";
+        WHERE sales_gl_code=" .db_escape($gl_code);

    if ($selected_id != -1)
        $sql .= " AND id!=".db_escape($selected_id);

=======================================================

For the moment, my simplification suits for my needs.

Generally, I believe, we have to take care that the combination of both (sales tax and purchase tax) is unique. There could also be the case, that the sales taxes has to be booked at the same account, but the purchase taxes have to be booked to the different accounts (for example in slovenia, when the same product is purchased in diffrent countries, but sold in a home country, even to the same client). What do you think about that?