If you are rendering an animation:
Render some frames on the laptop and some on the desktop, then combine the result files. Make sure you are rendering individual images not video. Render more frames on the faster machine and leave less for the slower one.
If you are rendering a single image:
Split your scene into 2 (or more) render layers, then render one/some layers on the laptop and the remaining one(s) on the desktop. AFterAfter rendering you can combine the result rendered layers into 1 image using Blender itself or some image editing tool (like GIMP). Again, render the more complicated render layer(s) on the faster machine and leave the less complicated one(s) for the slower machine.