SECN 2 is not mesh compatible with SECN 3 because they use different versions of Batman-adv which are not compatible.
If you have to run both MP1 and MP2 on the same mesh, you can use SECN 2 on both devices.
SECN 2 will not have some of the features of SECN 3 and is based on the older AA OpenWrt.
An alternative approach for a mixed network is to run two meshes, one for the MP1s and another for the MP2s, and link them together by connecting the LAN port of one of the MP1s to the LAN port of one of the MP2s i.e. run one node with an MP1 and MP2 back to back.