Neither of those products are thick enough to be called 'plate', so using the word may confuse your steel supplier... or cause the quoted price to go up.
Given curing time, RTV silicone can make a pretty good bond between those materials... but you didn't quantify how strong a bond you need. It probably wouldn't do well if you intend to form or punch the resulting composite.
Mike Halloran
Pembroke Pines, FL, USA