I don't think building a second march will have a point. All it would do is create a new area of unprotected and unfortified land. The current March covers enough of our border to provide good protection. If you want it to be stronger, direct actions toward it.
The only case in which establishing a March has great merit is in doing
@veekie 's triangle and then reabsorbing the March, which will no longer be greatly exposed. But even then, we could instead direct actions toward expanding their settlements in that direction.
Edit: I'd rather do our current trading post and colony options, but even then we only have 1 slot left until our prestige raises.
Edit 2: Yes, it would create more actions. But the actions they'd be doing would be the same as the current march, and with less strength than a strengthened march would perform them with.