I don't have any direct, first-hand experience, but I'll tell you what I know, for what it is worth:
1) there is a post here on the HUBB from a couple years ago about a guy trying to enter Russia with a vehicle either towing a moto or mounted on the back, etc., and IIRC he had a problem with the second vehicle; and
2) also a couple of years ago I was thinking about bringing in a second bike via temp import, and I happened to be at the border and asked the customs agent if it would be possible. They said that it was not possible to bring in two vehicles with temp import.
That said, I"ve also read ride reports about Poles, etc. that bring in a van full of bikes on their way to wherever, and they have apparently gotten in. So...you might want to track down the Poles (ride reports on ADVRider) and see what they have to say.