I get good coverage using 2 strands on 16 count. Sorry I can't help with the problem of the threads twisting. Possibly the strands are a bit long. I usually use one strand doubled up to about 18 inches, and then use the loop method of securing it to the back of the fabric.

There is really no set rule to follow but what I do

10,11 count= 4 threads full crosses, 2 threads backstitching

14,16,18= 2 threads full crosses, 1 thread backstitching

20 = 1 thread full cross, 1 thread backstitch

This is just my preferance others differ as to what they think looks best. Also most kits and alot of the charts will tell you in the key how many threads to use for the different stitches. Some will have different numbers for full and half stitches to give the picture a particular look.

As far as seperating threads I cut mine and then I hold the top of the group lightly in my left hand with my right I will take 1 thread and gently pull straight up. the remaining threads may bunch up a bit as you pull but once the single thread is seperated then they will fall back down.

I definitely agree on the 2 threads for 16 count and you may try dampening them ever so slightly with a wet towel to keep them from twisting, or use shorter lengths, or even use thread heaven on them.

I do what Rose does for separating the strands, and as for the number of strands I use, well that depends on the fabricbut also on the effect I'm after -- if I want it to look very delicate and watercolour-like, I might use one strand on 16 or 18ct, but if I wanted a bold, chunky effect I'd use two, and on 14ct even three. The thing to do is to experiment and see what you like!

I always use either one or two strands, I prefer the coverage that two gives but sometimes the pattern calls for one.

To stop the twisting I usually take one strand out of the skein, and let it hang for a few minutes and it naturally untwists, then I fold it in half to stitch with, occassionally I let the thread hang down with the needle on it and the weight will help it untwist while you are stitching.

Hope this helps

I think coffee_freak posted a tutorial on how to separate the strands:

http://www.crossstitchforum.com/viewtopic.php?f=3&t=8792&start=15

If the pattern doesn't specify number of strands to use, I usually double check the Cross Stitch Calculator to see what they suggest, and then test it out to see if I think the coverage is right.

I cut my threads no more than 40 cms then seperate them one strand at a time.

You can use this tutorial:
http://www.my-cross-stitch-software.com/faq_en/006/cross_stitch_006.shtml
to choose number of strands to canvas.

The secret of stopping thread twisting is to pull each thread individualy and then combine them back together. If using two strands I just make it longer and double it over and use the loop method for starting.