Suppose I had a phone that supports two SIM cards at once, would I be able to keep my parked SIM inserted to receive SMS?
It depends. Technically, as long as you don’t make any calls with your parked number (other than through the app) you can keep the SIM inserted and receive SMS while abroad. However, to make this work you will need to switch the primary card when you cross the border, otherwise the app will put you in roaming.
Even with that in mind, we still can't guarantee that this would work seamlessly, since Roamer is specifically tailored to work on single SIM phones.
If your dual SIM phone doesn’t have a primary SIM option, keeping both SIM cards abroad won’t work, you will have to remove the regular SIM card altogether.