There are two ways to go about using multiple hosts for better performance and uptime.
A load balancer accepts all connections for your website and forwards them to a pool of web servers. The load balancer keeps track of which servers are responding and which ones are not.
A hot spare is a website that is configured and running in a different location from your current website but which is not receiving any traffic. If your primary host goes down, you can switch DNS to point to the hot spare. This will involve some amount of down time because DNS records have a time to live (TTL) on them. I usually keep TTL to under an hour to enable this switch-over to happen in a reasonable amount of time.
If your website is running on shared hosting, neither of these may be appropriate for you. A load balancer with two hosts behind it is going to cost at least $150/month. A hot spare only eliminates long periods of downtime and is usually used with a load balancer in each location.
For much less than $150/month you could find a host that offers acceptable uptime. See: How to find web hosting that meets my requirements?