Z - table with combined primary key ( 2 fields ) ?

Hi friends,

i have a problem when writing an internal table to a z table that i created in ddic.

the internal table holds pairs and has structure as

i1 i2

1 1

1 2

1 3

2 2

2 4

and so on. The first question i have:

Which type of data does i1 i2 need to have in ddic when i use sy-tabix type for i1 i2 in abap ?

The next question then is:

If i have i1 and i2 marked primary key and i want to execute insert statement...it dumps.

Hope you can help me, i will award points generously!

Clemens