Editing this question to word it better.
I don't know if it's a unique dilemma or a silly premise, but it looks like I need more guidance than I thought. I'm not a history buff and kingdom/state dynamics are all new to me, but I still wanted to have a go at it.
Although the setting takes place in the 19th century, the universe is quite fairy tale, so the reality is an alternate one with many liberties and laws that only apply to its fantasy kingdoms. It nonetheless mirrors much of the 1707 Act of Union for the creation of Great Britain:
Country A is on the brink of bankruptcy and, much like Scotland, hopes to be pulled up by the more powerful Country B on account of being plugged into Country B's economy, international trade, and the like. Country B is their only parachute; there are no other willing saviors. Country B is also quite kipper about having some dominance over Country A.
But it is possible for these economic aspects to be shared through a union that is NOT a political one that unites two kingdoms and their parliaments? Like an economic union (Benelex Union), some form of devolution, a dual monarchy, or a commonwealth?
Or is the best way to become economically stronger as part of Country B's economy a political union?
(Wince) I should add that there is a marriage involved between the two monarchs reigning in both fantasy countries, who reign under constitutional monarchies. Coregency/joint sovereignty were RL things depending on the countries https://en.m.wikipedia.org/wiki/Coregency (rare situation, though), and in most cases the queen still wasn't equal to her hubby, except perhaps in the case of Isabella of Castile and Ferdinand of Aragon (?) or Mary I and Phillip II (their contract suggests joint reign that suppressed Phil), but I know a unitary state has never occurred unless a child inherited both crowns. However, since Country B wants control of Country A's bank management and government spending, I don't know what kind of Union could grant that power unless it's a political union.
For some background, monarch A and B, both childless and threatened by bloodline extinctions, (if neither have legit kids their enemies will seize power) are NOT heads of government, so with or without the marriage, the economic desires above still apply.
So there are two deals to be secured, but because I'm iffy about whether a full-fledged U.K. is reasonable (even if this is fantasy), I wondered if any other unions could guarantee the same equal privileges as a political union in regards to the economy boost, bank control, and sharing trade partners and treaties with foreign powers. If not, I guess I have to go with a political union if any of you would find that tolerable as a reader.
This scernario is still open for anyone to hop on. You guys have been brilliant so far!