Hmm... I would say one TREATY per order. If are signing it with multiple nations (for instance a multinational alliance) I think that should be fine. But not multiple treaties in one order, I don't think. Does that sound fair?
So to sign a treaty nation A needs an order of the type
"Sign a trade agreement to supply arms to nation B"
and nation B sends an order of the type
"Sign a trade agreement to buy weapons from nation A"
and you handle the rest?
