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If the second layer is a big sheet, I don't think you need to do anything.
If the second layer is going to be patchwork like the first, you are going to need to do something. If the reason for using 1" + 1/2" is that you want to create negative elevation effects, then I would do the carving first and use great foam or caulk.
If your reason is that is the foam you have on hand and you just want a flat surface, tape, caulk or great stuff would be fine.
I agree with the above comments on Great Foam.
Henry
COB Potomac & Northern
Shenandoah Valley
I would recommend 2 inch wide blue painter's tape. It's waterproof (painter's tape after all), strong, and doesn't deteriorate with time. It adheres to foam very well but it can be easily removed later, even much later, without tearing either itself or the foam apart. The resulting seam will be invisible after sceniking.
George
rrebell Latex caulk.
Latex caulk.
Perfect
Sound answer, sound advice. I would do what rrebell suggests, unless.....You plan to carve between layers. It's amazing how a brand new sharp razor blade reacts to soft foam with a layer of caulk in between.
If you plan to carve between two adjoining pieces of foam, the only thing that works in that situation is great stuff low expansion foam. I wouldn't get carried away with that stuff either. It comes out fast and less is more.
When adjoining bigger pieces of foam together, use screws until the aerosol foam is dry. You don't want the expansion foam to repel the other sheet
Never mess with the overkill! If it oozes out or gets on something, leave it alone and it'll come off like a dream after it dries. If you mess with that stuff while it's still wet, it will be there forever, dried like a hard plastic
TF
I am working on my first real layout. The first layer is 1inch foam and the second layer will be .5 inch foam. what should I fill in these cracks with? i think sculptamold might be too hard or bumpy. https://ibb.co/mv4PrqD