...a duel engine aircraft after we designed only one...
While it uses 2 radial engines, they can both be made smaller than the Emu's inline 8 and thus I don't anticipate a major cost increase for the added benefits. The fact that it's a fighter/bomber shouldn't matter - since it's a fighter first and foremost and geared as such, but it has the capability to ferry bombs...
Edit: Given bombs currently aren't at all large (our standard bomb stands at a puny 75 lb), making a fighter-bomber isn't difficult and doesn't infringe on the flight characteristics as much as you seem to be anticipating.
Information on the dogfighting ability of the Emu compared to the Emerald would've been welcome, but I think we can assume the Emu would hold it's own in a fight. What the White Victor brings to the table is a promise of higher speeds (I'd assume it could reach something like 180-200 km/h) while retaining good maneuverability and presenting a smaller target. The overall airframe shouldn't weight above 450 - 500 kg, even with the weight of the engines estimated at 100 kg (which is absurd for a 9-cylinder water-cooled radial), two engines bring 200 kg to the table, while the overall structure, using time-appropriate planes as a reference, should be around 500 kg when empty. This means we'd be dealing with a 700 kg aircraft (900 kg with fuel, passengers and ammo, assuming 100 kg as a norm for a human, something like 5 belts and 35 liters of fuel) with something akin to 240 hp (most likely less, something like 200 hp, due to interference).
Without going into aerodynamics, even assuming a drag coefficient of 0.05 based on
this, we're looking at something like 150 km/h with 4 m/s^2 of acceleration and an assumed weight of 900 kg, which seems accurate without checking average acceleration times for the period. After checking, the Nieuport had 2.25 m/s^2 of acceleration with a shoddier engine, meaning that if we match that, we're looking at roughly 265 km/h without drag - which fits with my earlier estimate, but without calculating for drag, which might reduce this to something like 190-170 km/h.
Given the shorter wings and plane in general, we can assume a much better roll rate as well. Given two 75 lb bombs we're looking at a reduction of 18.5 km/h without drag, so let's round that up to 20 km/h. We've now reached standard operating speeds of WW1 fighters WITH a bombload, and surpassed them considerably without one, all while carrying an impressive armament and possessing an edge in a dogfight. Keep in mind I grossly overweight this just as a proof of concept.
Basically, we'd be making a F.B.26 equivalent from the stats alone.
Anyhow. The production cost really depends on the dice. It could go anywhere between costing less than the Emu (by a bit) to costing double that, but I feel the performance would be worth it, and that at it's core, this is a plane that would cost about as much as the Emu.
Edit 2: Other than that, we could go for a proper bomber incorporating two 18-cylinder 2-row radial engines and try to get up to a load of something like 200-250 lb at 110 km/h. It would appear we're lacking in the bomber category right now, so we can't exactly make use of any air superiority, but while this would be effective, it would also be expensive to procure and maintain - which is problematic depending on what our budget is by the end of this. It's currently 22, with a cost of 5 for our main fighter/bomber and PP needing to be spent on dice and on maintaining what we already have at a certain rate I haven't checked yet (which comes out to 12 PP this turn), leaving us with just something like 7 PP for this turn. If you can buy dice with that, we could invest the other 2 into revisions, a new plane and what-not while spending 5 on a squadron of Emus.
Not to mention we're paying 2 PP for every Emu in operation, which is basically half it's production cost anyways! That's absolutely insane, as running costs are generally much lower than the purchasing cost for an aircraft. Next turn our maintenance costs would be 14 out of 22 PP. We should look into replacing some of our Rhinos with Emu squadrons, but then we risk being at a loss with planes, and operating costs would come out to 19 PP/turn if we turned all our Rhinos to Emus within the next few turns, leaving us with only 3 PP to conduct any kind of operations!