They're not doing it shoulder-to-shoulder, but they are doing the same thing in the same place. More evidence Syria was never an enemy to the US -- if anything the two share many a common interest
The war in Syria has seen quite a few interesting alignments. At one point the CIA-backed Islamists rebels were clashing with the Pentagon-trained SDF. At another both Russian and American jets were flying air support for a Turkish invasion into ISIS-held northern Syria. Russians flew air support for rebels against ISIS, and for the mainly US-backed SDF against ISIS. In cooperation with Turkey and the Saudis, the US showered the Islamist rebellion with arms, but on a few ocassion also bombed its largest and most effective component, the Syrian al-Qaeda, once with B-52s in a devastating raid. At one point the US was building up the Kurdish YPG to push back ISIS, while Turkey continued to tolerate ISIS resupplying and reinforcing itself across its border. At another Turkey was shelling not just the Kurdish, but also the Arab component of the US-backed SDF while it was battling al-Qaeda and its allies. At one point the Americans and the Kurds on one side and the Syrians and the Russians on the other found themselves both squeezing out ISIS from different sides even as the US repeatedly attacked the Syrians — and in one case Russian mercenaries in Syrian service — because they had allegedly come “too near” the Americans.
We can add one more “interesting alignment” to the collection. Just before New Year’s Eve after Trump announced the US military has been ordered to withdraw from Syria, hundreds of Syrian soldiers in agreement with the US-backed Kurdish YPG militia crossed over into Kurdish-held Manbij region of northern Syria. The purpose of the Syrians was to deter a threatened Turkish invasion and subsequent occupation of Manbij. This is to say Syrian soldiers are now in Manbij for the exact same purpose the American soldiers are there, the Syrians have effectively arrived to take over from the Americans and to guard their retreat and their Kurdish proxies. Since then the Russian military police has arrived for the same purpose as well, and the Kurds have slowed-down their withdrawal eastward across the Euphrates as conflict messages on US withdrawal come from Washington.
Video is said to show American patrol at Syrian Army checkpoint in Manbij countryside
This makes it difficult to predict what will happen next but the fact remains that for the time being there are now American and Syrian government soldiers positioned around the same small northern Syrian town, sharing roughly the same mission — the Americans to prevent a Turkish invasion from smashing the Kurdish-dominated SDF, and the Syrians to prevent the Turks from using the Kurdish excuse to expand their occupation of northern Syria and with it the reservation for the jihadists in their service. In other words, the Americans and the Syrians now find themselves on the same side, both facing down the same threat, NATO Turkey and trigger-happy Islamists on its payroll, if not together, then right next to each other.
In fact while the US has on almost dozen ocassions now struck the Syrian army there has never been an “exchange of fire” between the two. Though Americans are in Syria illegally and declared enemies of its government Assad troops have never fired on it. All instances where Syrians were bombed or shot down were instances where Americans proclaimed themselves to be threatened by the fact Syrians were so near (in some cases a full 50 kilometers away) and fired first. In one case they followed up the strikes against Syrians that were “too near” with wider, revenge attacks on Syrian formations further behind (it was in such “revenge” attacks that a dozen or so Russian mercenaries were killed — in their sleep). By contrast at one point the Americans around Kurdish-held Manbij were regularly coming under fire by the Turkish-backed Islamists that they themselves once helped sponsor, and only reluctantly and slowly got into the habit of firing back.