Hi chicy724,
I have no experience with putting one foundation on top of another and my concern would be that it wouldn’t be very stable or would be prone to shifting or noise with movement. I would probably want to make sure that you secured one foundation very securely to the other somehow so that they were very secure and there was no risk of shifting.
I think I would probably be tempted to use bed risers under your steel bedframe or taller legs and risers directly under the foundation along the lines of some of the options mentioned in post #8 here or post #8 here and use a combination that gives you the height you prefer.
Phoenix