Packing slip, more than one PO through x++

I'm trying to issue a purchase packing slip for more than one po (purchid) through x++ code

I''m using the salesformletter_packingslip, of course coupled with the parameter tables

So I pass the first purchid table and run the chooselines method, then I pass the 2nd one and run the chooselines again,

But it seems that the 2nd one overrides the 1st one and only the last po is received when the run method is called!

I tried to run the chooselines  only one time at the end, but I'm getting the same results

Can any one pass the code to do this task properly?

I know it's possible beacuse we can achieve the same functionality and choose different pos and select lines for receiving in the periodic form

Regards