You'll have to give up the idea of synchronizing with both environments. The underlying issue is that you want to violate the 'rule of conduits:' you cannot simultaneously synchronize the same data type with more than one conduit.
It is possible to do this only if you use the Missing Sync for Palm OS, which allows you to create separate conduit 'sets' into which you can place the otherwise conflicting conduits, then switch sets with a drop-down menu. You could do so using it to place the Palm Desktop and other conduits which do not conflict with them in one set, and place the iSync Palm Conduit and other conduits which do not conflict with it—and which can include any conduits other than the conflicting Palm conduits—in another. Or, you could toss iSync and the iSync Palm Conduit completely and use the Mark/Space Contacts, Events and Tasks conduits provided with that package.
You will never gain the substantial benefits of an advanced Palm device—a handheld organizer of smartphone running Palm OS 5.4 or later—until you ditch the m515. Clearly the Mark/Space conduits do a far better job than the iSync Palm Conduit does, but you need an advanced device to synchronize contact photographs, URL and IM identifier data, interact with iCal categories and Palm calendar colors, write back from your Palm device to any calendar which is not designated read-only on your computer, and so on.
In addition, the Palm Desktop is not now—and never will be—Sync Services compliant, so your data changes made in the Palm Desktop will not propagate to other sync clients, except in a limited and clumsy fashion. The Palm Desktop is for all intents and purposes an orphaned or abandoned product, and Palm is made it clear that it will not develop a Sync Services compatible release for future versions of OS X, and may not even offer maintenance releases beyond Mac OS X 10.4.9, so the sooner you walk away from it, the better of you'll be.
More information about the Missing Sync for Palm OS is available here:
http://www.markspace.com/missingsync_palmos.php