I not only don't tell them the save DCs, sometimes I don't even tell them what save they're rolling for, or even if it's a saving throw they're rolling. (It might be a Spot check, or a Listen check, or whatever.) "I need everyone to roll a d20," is sometimes all they get. With only two players (albeit four PCs), it's easy enough for me to do the math quickly and painlessly enough to come up with whether they were successful or not while leaving them completely in the dark as to what just happened.
Of course, in a situation where it's pretty obvious what the roll is for (like when a monstrous scorpion just struck a PC with its tail stinger), I'll 'fess up and ask for a Fortitude save or whatever - no need being sneaky when it's pointless to do so.
Johnathan