There are land border checkpoints. The Belarus side mostly doesn't check a thing, however the Russian side has started to perform checks, due to Belarus' new "5 days visa-free" rule.
Russia and Belarus do not recognize each others visas and do not have uniform immigration law. Even more so, those land checkpoints are not considered "international" checkpoints and crossing the land border is technically off-limits for international (as in not citizens of either Russia or Belarus) travelers even if they do have both visas. One land checkpoint does have "international" status, and that is the one that's located on the edge of three borders (Russia, Belarus and Ukraine).
Otherwise, travelling by air seems to be the only legal way to do it.
Since you don't need visas for either country, you might (or might not) be totally fine pulling that off though.