I have had an AMEX PLAT for as long as I can remember, probably since its introduction in Canada. I was also an Aeroplan E or SE when the Aeroplan AMEX Plat was introduced. So either I did not get an invitation for both for the price of one like you did or if I did, I discarded it without reading into it..
In any event, I did call AMEX and asked if I could subscribe to both for my current AMEX PLAT fee of $399.00 and the answer was no, I will have to pay for both. After some more discussions, they did offer me both BUT for a fee of $499.00. Given the signing bonus etc. I went for it and now have both the cards. The Maple leaf lounge access is a nice feature, now that I am retired and have no status with AE.