The Palo Alto  Firewall is a must and they have features that are not available with Meraki MX,  So we have to keep them and they are Managed by different organizations ( 3rd party), and the Meraki is managed by another Organization, So we have to keep them in parallel. 
 
I totally understand the design  is  complex  and I am in a situation I have to simplify the solution and keep both of them  MX/Palo Alto 
 
Please see below the Overall  Design. 

 
 
 
So what I am asking is how to simplify the Failover Mechanism between two Different Vendors with the Above Design. 
 
Any Idea you can share it with me?