So, let's see; Stalin reaches a seperate peace with Hitler sometime in early 1944 and likely before the Normandy invasion (Bargration and the Soviet 44 summer offensive would have only emboldened Stalin to continue given its success). For the Western Allies this would have been a MAJOR slap in the face after the Yalta conference. I have little doubt that the Soviets would have found themselves cutoff from lend-lease materials virtually immediately.
Such an agreement would also have reprecussions on the Pacific war. Does the Soviet Union declare war on Japan? If so, does the US grudgingly give lend-lease support? Would the Soviets declare war on Japan if the US withdrew this support?
If the Soviets do attack Japan they will very likely overrun a big chunk of China in short order. The Japanese are unlikely to surrender just because of this. Do US and US supported Chinese forces have relationship problems with the Soviets? For instance, the 20th AF flying B-24 and B-29s against targets in China and Japan might now be flying over Soviet controlled areas. How does the US coordinate their activities with the reticent Soviets (they were this way historically) who also view any aircraft and crews that emergancy land in their territory as virtually potential enemies?
In Europe the Western Allies likely would have continued the war to a conclusion. The Germans likely win the vaporized city award instead of the Japanese who come in a close second a bit later on. How much more mobilization would the US Army have to make to fight the majority of Germany's army later in 1944 and into 1945. Certainly, the US has the potential to raise far more divisions that they did but, this takes time.
What would the trade situation between Russia and Germany be? Would Stalin be willing to trade at all given that Germany was just a war with them? Do Romania, Hunguary, and other eastern nations that are allied with Germany continue to fight at all?
I suspect that the Allies might postpone their invasion for another few months while continuing their operations in Italy. Strategic bombing and the air campaign snuffs the Luftwaffe just as it did. The negligible contribuiton of Eastern Front aircraft making no difference.
Jets? By Jan - Feb 1945 both sides have useful flying models. The German ones have a pitifully short operational life due to poor manufacturing quality and lack of resources to make reliable jet engines.
Ballistic and guided missiles? The US could go ahead with a plan to fire 5000+ copies of the V-1 into Germany per month starting in October or November 1944 (yes, they actually considered this and Ford Motor Company was getting tooled up to do just this). The V-2 is surpassed in early 1945 by Convair's MX 774 while Germany's program is about maxed out at its original level.
The British and US manage by 1945 to achieve balance in the tank game with their M 26, M 34, Comet, and Centurian and related vehicles.
The ASW campaign still turns out in favor of the Allies. Yes, the Type XXI makes a dent, but the Allies improve their capabilities to counter it as well.
On the whole, the Western Allies still defeat Germany. Since Stalin stopped short of taking Poland and Eastern Europe these nations remain in the West's pocket rather than as Soviet puppet states.
China would likely still have a falling out with the Soviets by the 60's. Japan would still surrender to the US in the end. The Soviets lack the means to actually invade the Japanese home islands making their participation limited to China if the Soviets choose to participate in the Pacific war at all.
I see such an option as a losing situation for the Soviets all around. They are in a far weaker position postwar than they were historically.