One is a republic inside Russia that is formally ruled by Moscow. One is an independent country on Russia's western frontier that has long been Moscow's client state.
Vladimir Putin's Kremlin is accustomed to getting its way with both of them -- until recently, that is.
If you want a good barometer of how strong the Russian regime truly is at home and abroad, pay close attention to what happens with Tatarstan and in Belarus.
Because despite the consensus that Putin is the master of Russia's political universe and despite the spin and bluster about Moscow's resurrected great-power status, below the radar both Kazan and Minsk have been defying the Kremlin with stunning regularity -- and getting away with it.
In a recent interview with Gazeta.ru, political analyst and former Kremlin insider Gleb Pavlovsky said that 2016 will be a "terminal year" and a "moment of truth" for Putin's model of governance and statecraft.
"Incantations about a great Russia with dwindling resources and an obvious inability to manage what is left are accelerating the crisis. Everything is caving in," Pavlovsky said.
And in this sense Tatarstan and Belarus could be harbingers.
The Attributes Of Statehood
Tatarstan enjoyed broad autonomy under the presidency of Boris Yeltsin -- autonomy that was curbed significantly by Putin. But now it is trying to win it back.
Kazan has defied a federal law mandating that it stop calling its chief executive, Rustam Minnikhanov, "president." And it has flouted Kremlin orders to cut ties with Turkey in the wake of Moscow's conflict with Ankara.
Moreover, despite Moscow's protestations about separatism, not only does Tatarstan insist on having a president, it calls its legislature the State Council, has opened de facto consulates abroad, and maintains its own ties with a variety of countries including not only Turkey, but also Iran, Malaysia, Hungary, Kazakhstan, and Belarus.
The trend, and Kazan's refusal to reverse it, is increasingly alarming to the Kremlin and its supporters.
"Will Tatarstan Do To The Russian Federation What Ukraine Did To The U.S.S.R.?" political commentator Sergei Gupalo asked in a recent article.
Likewise, in a recent interview, the Kazan-based -- but pro-Moscow -- commentator Rais Suleymanov said, "Tatarstan is seeking to maintain as much as possible all the attributes of a state including the title of its head."
"If the revolution or Maidan that so many people are talking about takes place in Russia, if we have a repeat of 1991 in the form of the disintegration of Russia, then Tatarstan will have some all the institutions of a full-blown state and thus will be in a position to go its own free way," Suleymanov added.
In an apparent attempt to lay down a marker and send a message to the Kremlin, in December police in Kazan briefly detained Suleymanov.
It was a pretty bold move given that Suleymanov, who has harshly criticized Tatar authorities for failing to cut off ties with Turkey and accused them of fostering Islamic extremism, is widely believed to have ties to Russia's security services.
Writing on his blog Window on Eurasia, veteran Russia-watcher Paul Goble noted that the move was "an indication that Tatarstan has no intention of backing down either on its demand for the retention of a republic presidency or of breaking relations with Turkey as Moscow has demanded."
A Troublesome Client
And if Tatarstan is exposing the limits of Putin's clout at home, Belarus is flouting Moscow's will to a surprising degree abroad.
Most significantly, President Alyaksandr Lukashenka has thus far resisted Moscow's efforts to build a new Russian air base on Belarusian territory, despite intense pressure from the Kremlin.
One of the unexpected consequences of Russia's annexation of Crimea and proxy war in eastern Ukraine has been a chill in relations between Minsk and Moscow and a relative thaw in those between Belarus and the West.
Lukashenka has refused to recognize Crimea as part of Russia and even ridiculed Moscow's logic justifying the annexation, saying that Mongolia could just as easily lay claim to large swaths of Russian territory.
He has carved out a neutral stance on the conflict in the Donbas, has said he would never allow Belarusian territory to be used to attack another state, and has made it clear that Belarus isn't interested in being part of Putin's so-called "Russian World."
And even in Moscow's conflict with Turkey, Lukashenka has tried to maintain neutrality, calling on "our Russian and Turkish friends" to settle their dispute peacefully.
Moreover, after winning reelection in October, Lukashenka snubbed Putin by visiting Vietnam and Turkmenistan -- violating a long-standing tradition that his first foreign trip in a new term is to Moscow.
The new chilliness in the relations was evident when Lukashenka and Putin met in Moscow in December. Following the meeting, Putin coolly noted the "closeness" of Minsk's and Moscow's positions on Ukraine and Syria.
"In the language of diplomacy, phrases like 'the closeness of our positions' is common for countries Russia is friendly with, but not for its closest allies," political analyst Yury Drakakhrust of RFE/RL's Belarus Service wrote in a recent commentary.
"In a meeting between the Russian president and the leader of Brazil or India, it would be natural to note 'close positions.' But with a close ally, it is common to talk about the complete unity of positions, even if they are not in fact very close."
To be sure, Tatarstan will only push so far in its disputes with the Kremlin. And Belarus, which receives significant subsidies from Moscow, is not going to burn its bridges with Russia.
But both appear to be laying the groundwork for life after Putin.