It depends upon your ISPs, because the situation you describe is not a valid BGP configuration without a lot of cooperation between your two ISPs. The only way to load balance traffic into your site is to get your two ISPs to further cooperate to provide it. This may or may not be possible for them but will depend upon how they are currently faking the multihoming you are currently enjoying. There is nothing you can do without their help. Depending upon how the two ISPs work together to support your site, you may be able to load balance outbound traffic to the Internet from your site.
You may want to look into the RadWare Linkproof product. Its an applicance that claims to be able to load balance seperate ISP connections without the need to implement BGP. I believe that its does it via NAT. They also claim that it can load balance outbound and inbound traffic.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...