I don't know about substitutions, but I might suggest you talk to your doctor about the time of day you are taking your Singular. For allergies, it is recommended to take it in the morning. For asthma, it is recommended to take it in the evening for it to be most effective. I have been told this by both my doctor and my pharmacist. In fact, they now put a sticker on the prescription bottle to take it in the evenings.
Also, if you do continue to use Advair it is usually prescribed for twice a day. If that is how your doctor prescribed it, that is how you should take it for it to be effective. All dosages of Advair should be taken regularly for it to be most effective.