Well, I don't have a 100% definitive answer, but it is good enough for me: last weekend I had to take my bike out of Russia to renew the temporary import status. While I was bringing the bike back into Russia, I asked the customs official if I could bring in a second bike as a temporary import, and she replied with an emphatic "Nyet", and said that I would have to pay duties on the second bike.
Not sure what the regs actually say, but at the end of the day what matters is what the people at the border let you do, and at least at this border post (Latvia/Russia) they don't seem prepared to let in a second bike as temporary import. Could be different at other border posts I guess.
Also, no more temporary import certificates, at least at this border post--now they print out a sticker with the relevant data and paste it onto the customs declaration for the bike.