Hello Jagdish,
First of all I want to suggest you that please use the launchpad code instead of the debian packages because it will be easier.
2nd thing the complete command is nohup ./openerp-server & . if you are using this will start the services and it will not stop until you stopped the server. So as per my opinion please check the parameter again it will definitely works.
About your 2nd point for multiple instance the best way is as follow.
Create 2 different user i.e openerp1, openerp2.
download the code for both the user.
Create 2 superuser on postgres, openerp1 and openerp2.
Alter the role for both
then start the openerp server with different port via --netrpc-port=8090 --xmlrpc-port=8091 (You can also assign the host name, So you can use directly that url) . Then it will works great and your 2 different instance will be created.
If your services will be stop after sometime as you specified then do the one thing create the python script for that and it will run the openerp-server directly . So when your server will stop it will check auotmatically and start openerp services again. Here also you have to start your script with nohup command i.e nohup python script.py &
Thank You and hope this will helpful for you!