Ok I know I can jam the shield on my mega and it works, but in my aircraft instruments most of the time I need more than just 2x H-bridges. 
And the stupid things don't stack! in a very simple CDI you need:
Right - Left pointer (meter movement)
Up - down pointer " "
To - From Flag " "
2x Flag Motors " " PCM driven
Plus 2 for Chanel Resolver inputs
Pus a 400 hz Resolver drive
FUN
FUN
Oh I forgot to add that I use TX/RX#2 for RS-485