Here is what works for me when using bip32gen from bip32utils to generate the public keys which are used in the transaction, starting from an xprv master key.
For each xprv master key of a cosigner, use 0/0 for the first receiving address, 0/1 for the second, 1/0 for the first change address, 1/1 for the second change address.
Then use the corresponding public key for each cosigner, sorted into lexicographic order, when constructing the redeem script, and generate the corresponding p2sh address for each address which will be generated by Electrum.
So far as I can tell, there are no purpose codes or cosigner index, as per BIP44 or BIP45. This is with Electrum 3.2.3.
See this other stackexchange discussion: What BIP32 derivation path does Electrum use?