If you want to do an upload load-balancing scenario, you'll need two full routing tables or a default route / partial-route table at your edge borders; also an important advice if you want to deploy this scenario (do not use the rule drop invalid-tcp-connections at the forward/input firewall rule).
This is the answer you are looking for. You can't install defaults from both ISPs at the same time. If you pull full tables, your router will send traffic to the provider closest (smallest AS path) to your traffic's destination. This will not be true load balancing, but will keep traffic flowing out both providers.
If your router will not handle full tables from both providers, you can accept default routes (weighted to the provider with more bandwidth if they are not matched) and then filter out anything further than so many AS's. Just an FYI: at an AS Path length of 0-5, our table was around 5,500 routes from both of our two providers and can take your router some time to dig through and fully establish.