Hi. Hope You're doing well.
I've read your case, on 3 requirements you explained, unfortunately, you can't use it both simultaneously for supercharging your connection (like you have 2 1Gig links, it won't be 2gig but it still 2x1G either). There's a simple technique you can use for routing called ECMP / Equal cost multipath. It basically load-balance your request into 2 gateway based on weight or any metric u use.
(sidenote: ECMP only supported on Linux Kernel 4.17 (i believed) and later, if you use an older kernel, this may be not applicable. If you use an older kernel maybe you can use solutions like VRRP with multiple gateways.)
Let's have some chat on PM to discuss this.
Regards