Yes you get 250 on each.
You can prove it to yourself by logging onto .CA myebay and checking your used/left on .CA in the promotional offers section.
Then log onto .COM myebay and check your used/left under promotional offers section.
You'll see the full counts on both.
Time to get listing on .COM!!!!!
(and PS you are right that they have to be different items on each site, you can't sell two "copies" of the same item, one on .COM and the other on .CA or the bots will catch you).