Hello Tnarg and welcome to the forums!

1. cheap machine for 2 wows?

check this link for a cheap pc capable of running 2 wows:
cheap box - HP PAVILION SLIMLINE S3200N - $380

2. smallest res?

you can reduce the size to whatever you want, technically. whether or not it's playable is a different story. try to stay above 640x480 just because... keep in mind you can always reduce the fps to improve your machines performance

3. will software work across computers and then cross window?

yes... keyclone does this no problem

4. if so, how to do this?

buy a keyclone license for each computer, then connect the keyclones together... and add your wows to the keyclone window...

5. if i was feeling mad, could i run 4 on one machine?

yes... please run 5 on one machine with dual monitors. 2x2 on one screen.. and their main on another. check the screenshots forums for machines and specs

i hope that helps. have a great day!