Fun with routes

[aCaB <acabng@digitalfuture.it>]

I was trying to figure out a cheap solution to increase upload speed for
one of my customers (currently using ADSL).

I soon realized that dedicated lines such as CDN, HDSL etc are too
expensive in respect to common ADSL and come up with a brain-dead idea
of bounding various ADSL lines together...

Let's say I have ISP-A and ISP-B providing me 2 lines with static
addresses IP-A and IP-B; both ISP's allows source address spoofing.
For simplicity let's also say I will only accept incoming connections on 
IP-A, but, to increase upload speed, I want reply packets to come out 
both from ISP-A (with no modifications) AND ISP-B (with source address
spoofing).

I'm quite sure this can be done, but I'm not sure if iproute2 would do
this.

Any ideas?

I'm aware of legal concerns regarding IP spoofing, so that this idea is
quite unlikely to become a real life example, but I'd still like to know
about it for my own curiosity's sake.

Thanks a lot.