I would make Card B stronger and cost Card A at $5. That avoids people complaining about it being strictly better than Throne Room (yes I know it's not strictly better but people would still complain) while also making it more fun.
You're probably right about the people complaining part. I'm not sure about the more fun part though. Having it cost $4 instead of $5 means a lot of fun things:
- it becomes much more easily chainable
- it doesn't share a price point with $5 terminal draw, which makes engine building feel a lot better since you won't have as many of those awkward "Silver or nothing" early turns
- it emphasizes the fact that Card B is actually a junk card that you usually don't want in your deck
There's also very little room for Card B to be stronger, because it is still supposed to be a junk card. I briefly considered having them be like Ruins with different kinds of Duchess-like symmetrical effects, which would probably be more fun, but then I also liked this simpler version. I suppose I could make it just cost $5 or $6 to make the TfB interaction stronger, but that probably doesn't make the card strong enough to increase Card A's cost to $5 as well.