Multipath (?) routing problem

This is setup on Linux... not sure if that matters.... I don't know if this is a 'multipath' question or not...... I am trying to setup a new gateway on my network.... the existing setup is something like:

Internet -> WAN -> gateway-box-0 -> LAN -> local network. (active)

Internet -> WAN -> gateway-box-1 -> LAN -> local network. (in test)

Currently... ALL Internet traffic comes and goes through: gateway-box-0 AND my 'local network' has public ip addresses but my public subnet is routed through gateway-box-0.

The 2nd gateway... gateway-box-1 is setup and sort of works. If I change the default route on one of my Lan boxes to be

route add default gw gateway-box-1

and delete the main/working route

route delete default gw gateway-box-0

I can ping out via gateway-box-1 but any tcp traffic where it comes in on gateway-box-0 and goes out on gateway-box-1 does not work.

wget https://google.com for instance does not work.. the initial tcp request goes out via gateway-box-1 and the response comes in via gateway-box-0.... the source address for the request will be the same ip address but the path will be tcp traffic out on gateway-box-1 and tcp traffic in on gateway-box-0.

both gateways boxes see the box that the traffic starts on:

XXX.234.67.5ether 00:26:9E:58:67:76 C eth1

XXX.234.67.5ether 00:26:9e:58:67:76 C enp1s7

I've setup routing before where a box is multi-homed and has eth0 and eth1 and traffic that comes in on eth0 has to go out eth0 and traffic that comes in on eth1 has to go out on eth1... but in this case... all of the local network boxes just have eth0 and eth0 can talk to both gateway-box-0 and gateway-box-1 on the same network.

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Is there a simple TCP flag I can setup on the boxes in my local network that make them care less about multiple routes or is this something that just won't work?

- jack