What you can do here for u to not get any downtime at any time this is the best solution:

1. Have a duplicate site with same theme and stuff on 2 different shared servers or vps's which ever u prefer.

2. Have a external MySql database server get from a well known company that offers that and link your database to both the servers on the same.

3. Then on your NS records, try using a DNS Server such as everydns.net or any other that you like and add as follows:

ns1.sharedhosting1.com
ns2.sharedhosting1.com
ns1.sharedhosting2.com
ns2.sharedhosting2.com

Now give shared hosting1.com a higher priority and give the other less.

Incase one of your shared host goes down it will pick the site hosted on the other server.

This will make your site never to go down, this can only go down if your MySQL server goes down

Cheerz