The 2020 MSCA RISE Guide for applicants, on p. 18 of 75, states “Note that all secondments must be performed on a full-time basis.” In addition, the H2020 AMGA on both p. 528 and p. 529 states “The beneficiaries must respect the following recruitment and working conditions for the seconded staff member under the action: … ensure that the staff members are seconded full-time”. For part-time staff members this may require a change of their relationship (i.e. conclude a contract or a supplementary agreement), to allow the secondment to be implemented on a full-time basis.
The MSCA explained by your National Contact Points