Tropica Plant growth substrate unlike soil that is light and powdery goes like a firm clay layer with time.I have used it without separation and managed to separate 90%of the sand after 1 year without issues when I restarted the tank.
Just to update this thread for the kind people that replied.
I put a 1cm layer of substrate and then 3cm or so of sand down on top and so far almost everything is growing in nicely! My marsilea crenata carpet in particular is doing really well.
The only issue I had is that the schismattoglottis preteroi or whatever its called has rotted in places. Some has done well, some hasn't. I've bought more to replace the stuff that hasn't. In addition I've had a few grains of substrate pop up onto the sand because of trapped air bubbles escaping but this will likely solve itself in time.
All in all a total success and a good alternative to aquasoil if you only want to use sand