about offset question

in the table NAST there is a field named OBJKY

I want to use it in the table LIPS. but in the table LIPS there are two key fields.

if I want to select one line in the table LIPS, I need to know the two fields.

But in the table NAST, the field OBJKY contains the two fields together.e.g

in the NAST , the data of the field OBJKY :0080000002000010

but in the LIPS , the VBELN is 0080000002, the POSNR is 000010

so how can I slip the OBJKY to two fields?

select single VBELN POSNR

from LIPS

where VBELN = NAST-OBJKY(10)

and POSNR = NAST-OBJKY(11)+6.

I don't know if it is correct. So I need all you help

Thanks in advance

Nick