As a rule the (part of the) investigation carried out in Dutch territory must conform with Dutch law. However measures requested for, by an other State, can be executed as defined with the procedure of that State, provided such procedures are not contrary to the fundamental principles of Dutch law.
For the purpose of a JIT, participation of agents of the States, involved is a "conditio sine qua non". The concrete role and powers of the "foreign" agents on Dutch territory are to be agreed between the competent authorities of the States setting up the JIT. According to Dutch law foreign agents (too) are obliged to testify before a court or examining judge about their investigations, if so decided.