What would happen if I removed one of the valve springs....

I'm sure I'll get flamed for my ignorance but I'm in the middle of trying to swap valve stem seals on my KZ650. I'm a bit surprised by how stiff the valve springs are. It's making it tough to get the keepers off so I can do the swap. Theoretically, what would happen if I removed the inner spring and left the outer on each valve? Seems like there's plenty of tension to return the valve easily with just one....

Your valves would be kissing the pistons wrecking everything. 

Yeah bad idea.
Also I  recommend getting new stock springs because your bike if I remember correctly has very low miles and that means that some of those springs were compressed for 40 years and some of them weren't.
I think that your valves should be fine with just a clean up and a simple lap job.
Are you using one of these?    5 Sizes Valve Spring Compressor Pusher Automotive Tool Car Motorcycle | eBay
If not , they are a good investment, you don't want to bend a valve because it then isn't simple or cheap anymore.

Valve spring pressure isn't gauged by your hand.
That's what this tool is used for.
www.intercompracing.com/arbor-valve-spri...re-tester-p-105.html

And why do you think they used 2 springs instead of 1 stronger spring??
Harmonics, dampening, and valve float.
Valve spring seat pressure is only one part of that spring design.

www.cycleworld.com/2013/12/24/valve-cont...-3-by-kevin-cameron/
 

Draw a new redline at 5000 rpm and it's all good.

Am curious.  What are you using to compress the springs???........is the head out of engine??

I'm pretty sure Kawasaki knew what they were doing when they built these engines, as said above, removing any valve spring will end up in tears...  

PS, nothing wrong with your question, I'm glad you asked, get yourself a valve removal tool , or borrow one, Valve springs are very strong for a reason....

I may be little late but here are my experience.

Like many of us, I also work on cars. Pretty much any car but my favorites are American Classic. I can't afford real classic cars, so I myself drive 86' Monte Carlo SS. Anyway, one of the first thing many of us that work on classic American V-8 is like our bikes, change out carb, swap out the cam, put in different piston (to raise the compression) and such. But even from stock, these engines have rather weak valve springs. Usually with single springs with dampers. Without changing anything, changing the valve springs will raise the redline because valves stop floating. Now, I'm not saying by simply changing the valve springs will SAFELY raise the redline, but rather, it will stop the valve float at high rpm.

To this day, I haven't met anybody that put in a weaker springs or make the spring weaker just for the ease of the maintenance. Just look at it from different point. Just how often do you open or remove the head? Also, get yourself whatever tools necessary to do the job right. Flying valve springs are not fun, and it's very very dangerous.