Hi,
I am afraid that there might one some errors while creating PO.
For example, one purchaser is assigned to company A, and in its purchasing group, company B is also assigned in responsibility tab. However while creating PO, there is no field called company code to choose the one you want, it will always check the BUK attribute value assigned to this purchaser and choose the last one if no default value is set. If the account information such as cost center is assigned to another company, then the error "Purchasing across company codes is not allowed" will be raised.
I can suggest you one consulting note which enabled company code field in PO which should be of help.
1079525 - Select company code when creating a purchase order manually
Hope this helps anyway.
BR,
Ivy