2 Apple IDs - 90 day ban? I have 2 Apple IDs but I think I read that if you use a second ID, you cannot use the first again for 90 days. Is this true? If so, I need another solution!

95% of my app purchases have been with Apple ID #1, the original one. Apple ID #2 has only been used for a few important and more-expensive apps. I just want to log in occasionally to Apple ID #2 and download a purchased app, then go back to Apple ID #1 for all downloads, purchases, etc. 

Here is what we have. It's pretty simple because I do all the setup; my husband just uses the phone, iPad, etc. My husband and I have 2 iPhones and 1 iPad. He is still using a PC after all these years, but I have a Mac desktop with an iTunes account. Over the years I have created 2 Apple IDs.
Currently the 90 day restriction is only for automatic download of additional purchased content.

This also is a per device limitation and not something about the account being banned from working elsewhere. 

For 90 days after you turn on the "automatic download feature", you can't switch another account to start automatic downloads on that one device. You can always trigger updates manually. As long as you disable automatic downloads on the second account (or both) you won't have any issues with your two ID on multiple device setup until you run into the 10 total device limit for a given Apple ID for store purchases.