Suck it and see mirrors

Hi all i have a pair of suck it and see towing mirrors the type of which suction fit onto your mirror and adjust with the cars electric mechanism. 

I am not getting much luck with the flat mirror, the convex is great and just wondered any reason why i can t use two convex mirrors as when i tried it i could see brilliantly out of it especially on dual carriageways and motorways for lane changing.

Anyone use a pair of convex?

Thanks

Phil