You're welcome to try this. I don't have Evernote to test it directly, but I did test it with Skitch, which I believe is made by the same developers if we can draw any conclusions from that. Anyway, in Skitch, drawing a horizontal arrow is done, as in your case, by depressing the shift key whilst dragging the mouse cursor. Running this macro once successfully simulated the shift key being held, leaving my with one extra free hand whilst drawing a horizontal arrow having only to manipulate the trackpad. Running the macro a second time released the shift key.
Press & Hold Shift Key.kmmacros (31.9 KB)
I chose to bind it to my ⟨CAPS LOCK⟩ key, which may or may not be to your liking and is easily changed, but it made sense to me to do that. You'll need to set it yourself either way, as it's a hardware-dependent trigger, so I imagine it currently won't trigger on your system until you perform your own key binding.