Flat over semi-gloss - how do you do it?
It isn't so dangerous to do a flat on a semi if they both are good quality acrylic paints. It gets dicey when you put it over an oil the most.
Ive done what you describe with stripes ( same color, different sheens) and harlequin diamonds ( lots of layout!).
Gives an interesting and subtle look.
Cure depends on conditions ( humidity) and paint, but generally speaking about 3 weeks to total cure. If you paint before then, they bond to each other a bit better- but I think it should be fine anyway.
The flat won't be that cleanable, so do this in an area that won't get lots of abuse.
"It's better to come here with questions before you screw up than to come here after and ask how to fix them."- JS