Unfortunately, this is not possible — the order must be fulfilled item by item. See the Documentation for details.
For example, in the following way:
<winstrom version="1.0">
<objednavka-prijata>
<id>146</id>
<realizaceObj type="faktura-vydana">
<polozkyObchDokladu>
<polozka>
<cisRad>1</cisRad>
<mj>1</mj>
</polozka>
<polozka>
<cisRad>2</cisRad>
<mj>1</mj>
</polozka>
</polozkyObchDokladu>
</realizaceObj>
</objednavka-prijata>
</winstrom>
The </cisRad> element specifies the line number of the given item in the order.