"Awwww, look at the Germanics fighting the commies together."
Czechs are Slav. Flag's a clue.

Now, speaking of Slavs and solidarity, what I'd do to get something like what you're envisaging is to have Stalin back CZS to the hilt. Very unlikely, of course, given his paranoid fear of traps that Bruce has pointed out, but given that he actually was giving various kinds of overt support to the Czech position it's rather easier to justify than "Stalin invades Poland because yeah".
So, the Czechs tell Hitler to get out of it, and war breaks out in late 1938. The Czechs can hold the Germans off in their mountains for at least three weeks (that's actually a pretty pessimistic estimate, to be honest: remember that the Czech army was better than the Polish). In that time, the Soviets are casting about for a way to provide succour.
It would require considerable stupidity from all relevant parties, but this being hardly unprecedented in human history, it's possible (though again not
likely) that we could end up with Germany and Poland versus Russia and Czechoslovakia, what with Poland being considered a German client at the time. At the end of 3-4 weeks, the Czech lands are conquered, the Poles have been pushed back in the kresy despite false starts from the somewhat shambolic Red Army, and neither the exhausted German nor the ill-led Soviet army is in much of a state to engage the other. They glare at one-another over the Pripet marshes, and meanwhile Britain runs around giving guarantees to Romania, Finland, and Turkey.
The main difficulty is that France can end the war with one blow if it feels like it, but for narrative purposes I think we can have Chamberlain - who was almost as suspicious of the some Czech scheme to lure him into war on their behalf as Stalin, honestly, and was adament that he wouldn't go to war for France "in connection with her obligations to the country" - reign them in. The knowledge of British neutrality, and the "better Hitler than Stalin" attitudes of some in France, might plausibly leave us with a stalemate line drawn across Europe.