I've stacked them with no issues in the past, I think it says that you can't though in the t&cs.

However I will say, I have not had a SimplyMiles transaction post to my account in about 2 months, I'm currently owed around 8k LPs (~10k with the 20% bonus). Feels like something is broken at the back end. I'll shoot off some emails to follow up when I've downtime next week, but curious if this has affected anyone else?