If someone writes a detailed guide on how to have more than one masternode within a wallet ill send ya 100DRK for your service.
If you already have masternode wallets, dump the private keys for each one (dumpprivkey MASTERNODE_ADDRESS). Then on the wallet you want to have all the masternode in (make sure it is encrypted), import the private keys (importprivkey PRIVATE_KEY) for each masternode addressed dumped. Close the client. Create a masternode.conf file in the darkcoin data folder with the format below:
ip address of remote masternode:9999 MASTERNODE_GEN_KEY
ip address of remote masternode2:9999 MASTERNODE_GEN_KEY
Then load the new wallet with all the masternode addresses in there, go to the debug command prompt and issue:
masternode start-many PASSPHRASE
Not sure it can get any more detailed then that.