Here are about military export facts.
Good.Very good.
It might be prudent to ensure that China does not eat into the pie of Russia and rather concentrate on eating into the pie of other countries that are adversarial. Even if, on the short term, China may make a quick buck off on taking away from the Russian plate...on the long term, it will affect Russian Military Industrial complex which already is facing uphill climb/ low RnD spending and shrinking innovations. China can supplement and help Russian innovation and vice versa.
China ought to focus on selling more Surface Vessels ( type 056 corvettes) and Diesel Electric submarines ( Kilo might take a hit, but there could be some cooperation/ alleviation) and UAVs.
Russia can supply aircrafts and Land based systems.
Basically, both coutries need to figure out a clear path forward regarding building a strong portfolio of products that meet the needs of Defence market. It is doable. Competing against each other is ill advised. Both countries are in it for themselves and should have each other's back , whether they like it or not. Strong allies are hard to come by for China and Russia. I am afraid that there is none.