Hi,
I have a website and I use free hosting for the website. As it is free hosting I found my website down 5-6 time in a day. I dont have enough money to use paid hosting. So, is it possible to use another free hosting in addition to the existing hosting so that they work as back up for each other? I dont have enough knowledge about web hosting.
 

You get what you pay for, unfortunately. Most free hosting providers offer bad uptime or speeds just to convince people to upgrade to a paid plan. The only reliable free hosts I ever found were community hosts like Sorunome.de but then only old members usually can get hosting privileges and it's not guaranteed the host will exist forever (I lost my United-TI webspace twice before).

Soru, a community host ? Only Juju, you and I have stuff on his server besides him.

That's enough to call him a community host. Alberthrocks is also one with Withg.org. So if you hang around here and on our IRC you'll surely find a member with a server who will gladly give you some space on their server for free.

Otherwise I heard of 000webhost being not bad.

000webhost has bandwith limits for free websites hosting on them. But apart from that, it has a pretty decent uptime, so it is very suitable for websites that don't get a lot of visitors.
If you want a relyable server with good bandwith and uptime, you pretty much have to know someone who hosts a server and lets you use a part of his, or pay to rent a decent webserver. Alternatively, you could set up your own server at home. All you need is an old computer and internet.