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My apologies if this topic has been dealt with on this forum, but I haven't found the answer yet. I'm using an iMac and trying to drag the TVP GUI across 2 displays so that I can keep the timeline on it's own monitor.
When I get the cursor near the edge, I can see the horizontal arrow indicator appear, but then it's not allowing me to stretch the interface across to the other monitor.

What is the trick to doing this?

Technically, multiple displays are not yet supported by TVPaint but the workaround is to follow these guidelines:

In System Preferences make sure you do NOT have Mirror Displays checked.

You may already know that you cannot move any tool windows outside the main interface area. So you have to stretch the main interface across your multiple displays and then arrange the tool windows within this area. This of course includes the Timeline.

Another thing to be aware of is that the additional displays leave space where the main menu bar resides on your designated main screen. When you stretch the interface to the other screen(s) it will not reach the very top area. If you start with the interface on a secondary screen and have it filling this upper 1/4" it will not let you stretch the interface to the main screen. It is better to begin with the interface on the main display and just pull on a side rather than a corner so at first you are only widening the interface. Then tweak on the corners or just the top and bottom edges after you have adjusted the width.

When you stretch the interface across multiple screens make sure that you leave 1/16"-1/8" space on every edge. Do not allow the interface to touch the edge of the displays or else TVPaint will not recall the arrangement and default back to a single screen when you close and reopen the software. It is OK if the top of the interface touches the main menu bar of the OS.

Once you have everything the way you want it as long as you have followed these guidelines TVPaint will faithfully restore your multi-display setup.

Note while using a multi-display spread:
You can use Show Full Page and the drawing/camera/scene area will dominate the main screen and keep your multi-display spread.
If you select Full Screen, TVPaint will collapse back to the main display and your secondary display(s) will be blacked out. Use F11 to exit. If you use the esc key you will end up with everything collapsed and piled up on the main screen.
Show Full Page and Full Screen produce the same sized working area. Between the two use Show Full Page so you have all of your tools available on the secondary display(s).

Thanks oOAmpyOo! The issue that I'm encountering though, is that the GUI isn't "stretching". When I drag one edge, the opposite edge doesn't stay put, but rather follows. I'm just dragging the entire interface around. How do you get it to stretch?

Hmm That is something else I guess.

Does it behave the same way whether pulling a side or a corner?

... hold the presses! For some reason, it just worked! I have my 2nd monitor above my Cintiq and I was able to stretch the interface up through the top monitor. The issue that I'm seeing now is that I want to keep the left side windows (Menu Bar, Color Panel, Main Panel, Tool Panel) on the lower screen with the Project window, leaving just the timeline in the upper monitor. But when I drag them down to the Cintiq screen, they just snap back up to the upper monitor when I let go of the mouse button.

Is there a way to make them stay down on the Cintiq alongside the project window?

Which version of TVPaint are you using?

I have the freedom to place everything where I want them within the main interface.

I am using two identical displays and not a Cintiq though.

Try turning on/off "Magnet"" (Preferences => Display => Panel => Magnet)

Did you make sure Mirror Displays was unchecked?

I'm using TVP Pro 11.0.6 - 64 bit

I tried what you suggested with unchecking the "Magnet" box, but it's still doing the same thing. After I drag the panels to the Cintiq and let go of the click, they shoot back up to the top monitor.

Did you make sure Mirror Displays is unchecked?

Yes, I have "Mirror Displays" unchecked.
This is a puzzler!

oOAmpyOo, do you work on a Mac?

Yes I am running OS X 10.10.5 Yosemite. I have the same version of TVPaint as you have. Maybe you could try swapping screens between which is designated the main and which the secondary to at least see how things respond. This can be done with the simple check of a box in Preferences. If it is the same except swapped it still provides more information to consider.

I'm running 10.12.5 Sierra. Do you suppose that could be the difference?

It is possible.

... something else I just noticed... when I drag the panels down to the project window area, they stay down there. But as soon as I slide them to the left, into the lighter gray vertical area that they dock in, they scoot back up to the top monitor.

You have to go to System Preferences > Mission Control > Uncheck "Displays have separate Spaces". Then restart your Mac.