pants on circulars? converting to straight needle?

[sinistermommy]

Hi everyone, this my first post here. =) Nice to find a board and interaction instead of just spending time surfing through 'how to' sites.

Anyway, I have been mulling over something for a few days and I'm not sure what to do next so I thought i would ask here.

I have a pair of pants I want to make for my son, from a pattern I found for shorts. It will hopefully look a lot like this, sans pockets:

when I'm done with it. (our scanner's down so I can't scan the original pattern picture, sorry)

The pattern is really really simple (except for the part I'm stuck on), and worked from top down.

Cast on 136 stitches, then work in k1p1 for 12 rows, then next round increase 1 st in every other st until there are 204 st.
Continue knitting til pants measure 8.5".

Crotch shaping:
divide sts into two equal sections. Knitting back and forth on the needles, dec 1 st at each edge, every other row. Cont. until pants measure 10.5".

Cuff: k2tog every other st across row. Work 5 rows of seed st for each cuff. BO.

Ok, so that's simple. But I generally don't work on circulars, and I'm unsure about the leg part. I dont' understand how you can work both legs at the same time on the circulars? There is a different pattern in this book which says to do so.
Is it so simple that I'm missing something? Or do you actually knit back and forth as on regular straight needles (thus the 'knitting back and forth on the needles')? In that case, you would do one leg and then the other, right? But then I would have a seam.

Is it actually possible to have completely seamless pants by knitting in the round without working on each leg separately? On the wooly wonder pants pattern, it says that it's knit in the round with no seams, both legs simultaneously, but I cannot fathom how that is possible.

That brings me to the other part of my question. I've only ever used circulars to make blankets, and my needle is 32" long. I tested this pattern by casting on, and then knitting the ribbing and a few rows beyond, and these pants were gigantic, far too big for my son even though the gauge with my yarn for this is pretty right on, and so I decreased the stitches to CO about 80. I had to rip it out because I just couldn't quite get the knitting to wrap around the length of my needle. *sigh* Obviously I need a 16" needle. Although if I switch to a straight needle (I only have size 11 and size 8 circulars) I can go to a 5 or 6 to 'overcorrect' the gauge.

So. I've decided that I want to continue this pattern anyway, at least to see how it turns out with all my modifications. Though I'd prefer for it to remain seamless, I can deal with a seam or two, so I can make it on straight needles and not have to go buy another circular since we're super short on $ right now.

Now, I think this pattern would be pretty darn easy to convert to straight needles, but I'm not that good at looking at a pattern and visualizing how it works, so I was hoping someone could help me with that.

Obviously it would be easy to just make the seam outside one of the legs, but I'd rather have the seam on the inseam and down either the front or back of the crotch. I haven't really sat down and tried to plan it out yet, but I'm going to make myself a picture. Is there someone better at this than i am, who can help me convert this pattern, or at least help me proofread it? I'm mainly just a little wary of the crotch shaping and screwing that up when I change the pattern.