Probably you won't need more than some KY jelly.
Never connect two drain hoses using a tight Y connector otherwise one machine can drain into the other.
Probably both hoses are quite flexible so you can put them together in the same pipe (according to it's diameter) and they are going to be only a few milimeters tightened, so it won't afect the flow. The KY jelly is water based so won't harm plastics or rubber, later, it will evaporate and dissapear completelly. Lube both hoses and the internal side of the pipe and use some elbow grease to slide both hoses in the pipe at the same time. (i did it here many times to make two hoses fit a 40mm stand pipe) If your hoses are the thinner type that most european washers have, you won't need to do anything, just put them in the pipe.
If your stand pipe's diameter is wider than that, it will be evten easier.
Only be sure if the pipe can stand both machines draining at the same time. Do a small test: Fill both machine swith water only and set them to drain. If the pipe starts to overflow, you'll notice and have time to turn them off before no more than some mililiters of water fall on the floor. As european front loader drain pumps aren't too powerful, i bet it's virtually impossible to have some kind of overflow with only two machines but it's worth testing.
About the Y connectors to fill the, you can put as many as needed. It won't leak if you do the things right. And even if it leaks, it's going to be only a few drops, so you can notice it almost instantly and retighten or remake the connections on the leaking point.
If you get a 3rd, 4th or whatever the number of machines in the future, the process will also the the same, more Y connectors, etc.
Again i remind you to be careful about the wattage on both machines. Check if the outlets can stand two machines. It's almost impossible to start a fire, but it can happen if you ignore this. If you use both machines and the circuit breaker goes off or the fuses blow, it means you have an overload and the wires in your house aren't strong enough to stand such amps. In this case, you only have to reset the circuit breaker or replace the fuse and everything is going to be fine again. If this happens, you can even keep both machines connected all the time to the power outlets, but never use them at the same time.
Finally, both machines must have an own outlet. If you have only one outlet, it's very easy to replace it (we can teach you). NEVER, i mean REALLY NEVER use adapters or hubs to connect two mashing machines to the same power outlet. Those adapters are ok for two or even three very light electronics like a cell phone charger, your digital camera battery charger and your printer in the same outlet, for example, but never two washing machines with internal heaters.
How high is your water pressure? If it's too low, things can be a little difficult to the dispensers when using powders, but this situation can easily be solved by starting one of the machines first and wait until it's full (1 or 2 minutes maybe) then start the second machine.
RIght now I can't remember exactly how many Y connectors I have here. One thing I'm sure is that I have more than 9.