Stacking two front load washing machines on top of each other is generally not recommended and not safe. Front load washing machines are not designed to be stacked in the same way that front load dryers are.
When it comes to laundry appliances, manufacturers often design specific models for stacking, such as front load washers with matching front load dryers. These machines come with stacking kits or brackets that ensure they are securely fastened together and won't pose a safety risk.
However, stacking two front load washing machines without the proper stacking kit or design could lead to instability, potential damage to the machines, and, most importantly, pose a significant safety hazard. The top machine could easily become unbalanced during the spin cycle, causing it to tip over or fall, potentially injuring someone nearby.
If you need to save space and want to stack your laundry appliances, you should look for a matching front load washer and dryer set designed for stacking, and make sure to follow the manufacturer's instructions and use the appropriate stacking kit. If you're unsure about your specific models, it's best to consult the manufacturer's guidelines or contact their customer support for advice.