If I understand you correctly you want this:
now as user83046 suspected is true you will get a full loop (so you can keep moving the mouse to the right and it will loop around all the monitors.)
BUT to do it you will need to edit the config file manually. I think some of the older config editors could do it but the new one is really bad (or to complex for me! ;-)) you didn't mention what OS your using so I will assume Windows on both PCs.
First what you want to achieve is closer to this ("<-->" = Synergy's Links. [...] is a monitor):
Notice how Desktop2 is in two places? The "Desktop2" on the far left is where windows THINKS it is and the one on the right is where you have really put it!
How to accomplish this:
1) Setup your monitors physically like this:
2) Tell MS Windows on the Desktop that your 2 desktop monitors are like this:
3) Then with notepad or your favorite text editor (I use Notepad++) create a new file called Synergy.sgc and make it look like this (I have assumed you have named your laptop "Laptop" and your two screen desktop "Desktop" in Synergy:
left = Laptop
right = Laptop
right = Desktop
left = Desktop
4) Save the file somewhere easy to find and where it can stay permanently (with an extension of ".sgc").
5) Now from the server (the PC with the mouse and keyboard) open Synergy.
6) Ensure "Server (share this computer's mouse and keyboard):" is ticked and select "Use existing configuration:"
7) Select the config file you just created (Use the "Browse..." button to make sure you get it correct).
8) Click Start on Synergy on both PCs
This should now let you use the monitors like you wanted!
Note: To move a window onto [Desktop2] you MUST drag it right from [Desktop1] (remember how windows THINKS your montors are setup)!
BUT as mentioned earlier when you scroll of the right of your Physical right monitor "[Desktop2]" your mouse will appear on the left of "[Desktop1]" which can be confusing! LOL
(Everything below here isn't needed for the above solution but might help you get more out of Synergy)
I have tried before some more advanced methods to remove this loop around affect by staggering [Desktop1] and [Desktop2] like this:
to make it so you can only cross between the two monitors in one corner but Synergy just prepends there is a monitor there and lets your mouse go into the (Void) below [Desktop2] and that was just even more confusing! LOL
The other option you could use to reduce the wrap around affect if it is getting on you nervs is to tell windows the desktop monitors are like this:
and then instead of wrapping of the edge of the two outside monitors you could go up on on and apear on the bottom of the one on the oposite side... Can't see this being any more useful BUT might help you get some ideas for other setups of your own... ;) I personally use 3 24" monitors and a 50" plasma on two PCs to get this affect:
| PLASMA |
| PC1 |
_______| 2 |_________
| PC1 | PC2 | PC2 |
Which is the plasma and Monitor1 in Landscape on PC1, Monitor 2 in portrate and Monitor 3 in landscape on PC2... (encase you think the config will help you with your setup its below)
left(35,100) = PC1
up(0,36) = PC1(22,40)
right = PC2(35,100)
up(70,100) = PC1(0,22)
down(0,22) = PC1(70,100)
down(22,40) = PC2(0,36)
Hope that helps someone! ;)