2 Psu in one PC hepl

Jeesh, I suggest you do not play with your PSU units unless you really know what you are doing! linking them in this manner is not advisable, if you need more juice, get 1 higher rated PSU.

You should not modify a PSU like that EVER. If you need more power spend the 100 and get a better PSU, the average person does not have the means to appropriately join wires that will have that level of current passing through them, the wires in your PSU carry alot more current than your household wiring meaning that a poor connection will generate significantly more heat than one in your house would. If you do this you void every warranty, of every component in your case, and risk shorting out a wire and having the PSU expode.

Now which of these seems the least expensive?
1. Buy a good large PSU for 100-140
2. Buy an entirely new computer
3. Pay for funeral expenses because your hack job exploded and killed you.

If you think the third is unlikely you have never wired an electrolytic capacity backwards, they go off like little rockets.

or buy one of those antec cases that allows you to link two psu's

I have a case that I have been using for 3 years it came with an adaptor so that when I press the power button both supplies are turned on. I can't remember the manufacturer, I stumbled on your question looking for a new case. I remember seeing only 2 wires connecting to the 20/24 pin adaptor. I am sure that there are wiring diagrams or you should be able to find an adaptor online. Don't worry or listen to those that say "Oh never do that" they are the same people that pay $50 to have a stick of ram installed. Good luck on your search, I have been looking for two days. With the SLI setups 2 power supplies are becoming more common, they make power supplies that fit in front bays.

Just because you can do something doesn't mean you should.