you can not bundle, but you can load-balance between those Links.
An option you can split the load like part of the user network to use WAN1, others WAN2 so on.
you can do with PBR - also using IP SLA you an failover to other using IP SLA.
here is the good thread for your reference :
https://community.cisco.com/t5/network-security/configure-pbr-with-ip-sla-on-ftd/td-p/3033646
