Welcome to the Fortinet community.
A few ideas on your scenario.
1. It is complicated...
In most cases one of the providers is "preferred" as being cheaper, faster, more reliable, and so on. In that case you would use AS prepend, for example to manipulate the incoming traffic. You would perpend your as to the "less" preferred ISP so upstream ISPs will receive the prefix with a longer AS PATH from it so will not preferer that path.
More info about this can be found at: Technical Tip: How to configure BGP AS prepending - Fortinet Community
If, both ISPs are preferred than it depends on the ISPs and their peering. Fore example your client has ISP1 and ISP2 and a customer from ISP3 wants to reach your client. How will it reach it? It depends on ISP3 and how it prefers to route traffic towards the "Internet" over ISP1, ISP2 or a completely different ISP. So, even if you prefer both ISPs they might have different routing strategies.
2. Can you be more specific? If you use BGP as routing protocol you would have to advertise the prefix to both ISP. Also via BGP you will, supposedly, receive the default route.
3. Yes, you can rely solely on static routes for both incoming and outgoing traffic.
As the question are relatively general, so are the answers. However, If you have specific ones do not hesitate to post them here.