The havoc wrought upon Union shipping by the seven CSN commerce raiders operating up to October 1863 is a good indication of the potential of the two Russian squadrons, if employed individually against the British merchant marine in the Atlantic and Pacific. (This was presented well enough at
http://civilwartalk.com/threads/russian-fleet-shows-support.9760/#post-117349.) There was only a thin tissue of naval strength provided by the Royal Navy on the coasts of China and India. The Navy Lists show forces adequate to suppress piracy and intimidate the natives, but not up to either pursuing raiders with auxiliary screw propulsion, or to convoying the merchant marine against their depredations. Most of the warships in these vital trade areas were low-power (slow) gunboats run up for the Crimean War. A core of faster, well-armed screw sloops was scattered very thinly across the Empire, and as Union pursuit cruisers had found, the seas are wide.
The Russians would contribute to the ruin of British commerce, the fear of which most affected that nation's American policy and strategy.
Assume that breaking the federal blockade of the South would be the first British/French step -- to feed/re-equip the Confederate armies and to stoke up the ailing English textile industry. The best response by the Union would likely be to disperse all gunboats with "legs" (leaving out the paddle steamers and shallow-draft propellors) to North and South Atlantic, Pacific, and Indian Oceans. Union raiders would outnumber Royal Navy defenders. For a few weeks or months, Pennsylvania coal might be gotten to sea to supplement the larger at-sea supply carried by British colliers in trade around both southern capes.
The USN also had few warships in the Pacific, but it had yards in California and access to some middle-quality coal at Whatcom, Puget Sound. I'd guess that British entry to the war would trigger an immediate scramble to knock out opposing Pacific naval bases and shipyards: at Esquimalt and at Mare Island/San Francisco. With not a single ironclad on that coast, the San Francisco forts might be potent enough to free up USN ships to take Esquimalt (which had no artillery defenses except those carried by warships in harbor) and the superior coal mines at Nanaimo. Of course, no further gold shipments could safely be sent via Panama, and delays would cause financial problems (possibly grave crises) in both New York and Washington. On the other hand, protecting New Zealand gold shipments to India would draw off some Royal Navy strength.
The Royal Navy was impressive in home waters where the ironclads could operate, but would have been wholly ineffective in protecting British commerce against the numbers of raiders which the Union could release from blockade duty. France and Britain posed no more real military threat to the United States than the US posed a naval threat to either. But the joint ironclad fleets could have threatened New York and other ports. That circumstance would have provided a telling test of the two ironclad design theories, since to be useful at all in the Americas, the British/French ironclads would have been forced to fight the inshore battle that monitors were best suited for.
It's just a theory...
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The Russian Empire would probably do everything possible to draw the Union into the war... and visa versa, actually. I think, if the war were declared between Union and Britain, the Union would do everything to draw the Russia into the war. After all - we were the only possible ally.
Bringing the