@kittyniegh: I'm glad you like it.
@fire_heart: I'm probably going to make a tutorial about this (a lot of people have been asking).
First of all I'm going to assume you understand layers if you (or anyone else reading this) does not understand please feel free to message me.
Step 1:
Select the layer containing the lineart (the selected layer is blue in the layers pallette). To double check you have the correct layer click the black dot to the left of the name. This should make the lineart disappear. If there is shading as well you need to "merge" the lineart onto the shading. If you do not you will have one creature that is shaded and one that is not which looks odd. To merge you go to the top bar select layers and click "merge down". NOTE: Make sure you have the upper layer selected, otherwise you may merge your lines to the background.
Step 2.
Duplicate the layer TWICE by going to the top bar, clicking effects, and selecting "Flip Horizontally". (You can flip it vertically as well, it just doesn't work quite as often). Hide one of the layers for now.
Step 3.
Set both layers to multiply. To do this go to the layers pallette and click on the arrow by the word "Normal". There should be a long list. Select Multiply (near the top). Now you should see both of your linearts overlapping and the background behind them (if it is a checkerboard I would suggest putting a white background. When I color I use bright red, but it's up to you).
Step 4.
Move the lines to where you want them. To do this use the move tool (looks like a compass rose) and move the lineart a bit at a time. Please note: If you move the lineart off of the screen and let go then that part of the lines will disapper. You can use the CTRL+Z keyboard shortcut or the top bar, edit, undo if you accidently delete part of the image.
Step 5.
Decide which one you want in front. Use the eraser tool to remove the other lines. This can be confusing, (For example look at all those legs!), but you can hide/unhide them to check and make sure you are doing it right.
Step 6.
Remember the layer we hid? That was so that you wouldn't accidentally delete the artist's signature. Go ahead and hide the other two lines, show the signature and erase the creature from it. Set that layer to multiply.
Step 7.
Show all the layers, you should have two critters and the artist's signature. Add your own signature to the signature layer, make a new layer below all of the lines layers, and start coloring!
Please feel free to PM me with any questions or clarifications.