I have an experiment with 4 conditions: control1, control2, treatment1, and treatment 2. Each has approximately the same number of observations. I want to perform a proportions test to compare the number of successes in each group. To increase the number of observations, and decrease the number of comparisons I run, I'd like to combine the two controls into a single control with 2x the observations. Is this good statistical practice?
First of all note for the record that it is pointless to test the difference between control1 and control2 since if you randomised them you know for sure that any differences are due to chance unless you have reason to believe that the randomisation process was subverted in some way. I know you did not say you were planning to do that.
Other than that there seems no reason not to combine the two groups and use them as a single control group.