You need an iBGP session between your two border routers and a way to provide a default gateway to internal network.
One possible way is to use HSRP to provide a next-hop to the ASA pair as explained in the document above.
Another possible option is to use an IGP EIGRP or OSPF to generate a default route in the IGP from the border routers and to have it propagated via the ASA to the internal network to other network devices ( this means running the IGP also on the ASA pair).
Also consider for the BGP multihoming part the following doc that shows the various options (internet full table, only default route or default route + selected routes)
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