From a long while back, but I forgot to add it as a blog post.
The answer to [Wayback/Archive] delphi – How to have both VCL and FMX in one application? – Stack Overflow (thanks [Wayback/Archive] Gad D Lord for asking and [Wayback/Archive] Aleksey Timohin for commenting) is actually straightforward so Gad wrote a blog post on it back then [Wayback/Archive] MTG Studio: How to create and application which compiles both for Firemonkey and VCL.
It follows my answer closely, so here it is:
The Wiert Corner - irregular stream of stuff 