Not very high, I'm afraid. I have never hatched anything myself, but from reading this forum many first timers have problems. One of yours would most likely be that since you want only two chicks you'd go for one of the small, cheap incubators and those tend to have trouble keeping the temp steady. You'd also need an extra thermometer and hydrometer to make sure the built in ones aren't way off. If there even is a built in hydrometer.
However I'd actually suggest coturnix since you want pets. Two female coturnix can live happily together and they are easier to tame than buttons. Buttons can be tamed, but it requires that you spend hours doing so every day from they hatch till they grow up.
Keep in mind that statistically speaking you'd ned 4 eggs to get 2 females, but you could be unlucky and get 4 males - if all 4 eggs hatch. So I'd go for 10 eggs. With a 50 % hatch rate that would give you a decent chance of getting 2 females - but to achieve a 50 % hatch rate you do need to have both good eggs, a decent incubator and to do everything right.