NAT out to two DSL modems

Hi,
I'm trying to NAT out to two DSL modems.
I have three network cards on three subnets:
re0: 192.168.4.0/24         Internal
re1: 41.134.100.222/29    DSL_A
re2: 10.10.10.5/24           DSL_B

I can NAT out to either re1 or re2, but I have to make my default
gateway point to the relevant gateway on that network.
How can I tell the route tables or the nat-to command what the gateway
machine is?

So I can do this, but ***only if my default gateway is
41.134.100.217*** (which is the gateway for that net):
pass out on re1 proto tcp from 192.168.4.0/24 to any nat-to re1

Likewise, I can do this, but once again, ***only if my default gateway
is 10.10.10.1*** (which is the gateway for that net):
pass out on re2 proto tcp from 192.168.4.0/24 to any nat-to re2

I believe I should be able to make this work without ANY default
gateway. But then where do I tell the system
what these two gateway machines are?

Thanks,
Ben