Well, thank you for the more answers :>
Jervig, due to university related stuff we cannot be sure, if the year afterwards we will have enough time to do a trip at all. Unfortunately
Daze, actually what you describe is almost exactly what we plan to do by now

. We are just not sure yet, if we go through iran and Turkmenistan (Carnet is expensive) or take the ferry from Baku to Aktau (time schedule not reliable), or if we go from Azerbaijan to Russia and then Kazakhstan and then south through uzbekistan (safety issues in North Caucasus). "Worst" case we enter russia from Latvia and then go South. We will see what time brings (financial aspects, safety situation in different areas, etc.). There are many opportunities for the final route. We also still need to do a lot more research.
We like the idea of selling the bikes in mongolia and flying back, because after mailing with the customs it turns out that there is not much of paperwork problems

.
We are going on a Honda Transalp and a Dominator, which we think shouldnt be a problem.
Interesting is the aspet of time issues that you mention, which is also one of our bigger concerns. But we "only" have to average a little more than 300kms per day, so we thought especially because in the first few days in Europe we can make a lot more than that already, and if we plan our riding and especially resting times well, the time schedule should be okay - at least we expect that right now^^.
Daze, was it hard to find someone to sell the bike to in Ulaan Bataar? Did you have to sell it far under value?