Just wanted to share my experience travelling overland from Belarus to Russia. I travelled from Minsk to Moscow by train and didn't get any stamps when crossing the border, because there's no border control between Belarus and Russia. This means I didn't get an exit stamp on my Belarusian visa and no entry stamp on my Russian visa.
Turns out, this is not a problem (but I couldn't really find any definitive information about this on the web). When I exited Russia (over land) I just got an exit stamp on my Russian visa and got no questions asked. Do keep the immigration form that you got while entering Belarus, as they will stamp it and take it when exiting Russia.
