Place each chest with at least one full block of space between them
Use single chests instead of placing two chests directly next to each other
Put a solid block between chests to prevent them from merging
Place chests facing different directions if needed
Use trapped chests only if you do not want them to combine with normal chests
Avoid placing a chest immediately beside another chest on the same side
In Java Edition, use a barrier, slab, or other block to separate them
In Bedrock Edition, keep chests from touching side by side
Use shulker boxes or barrels if you want separate storage without combining
Place chests diagonally instead of directly adjacent to each other