Using an AC splitter for A51m?

So just to be clear, you are NOT trying to use a single AC adapter to feed both of the "inputs" on the A51m?  You're instead still using two separate AC adapters to feed the A51m and you just want to use a splitter on the wall outlet?  That's completely fine.  Many household appliances draw more power than an A51m, such as microwave ovens and vacuum cleaners, and individual wall outlets do not have any sort of intelligence to directly limit the power output they provide -- which is why splitters and surge protectors are feasible.  The only thing that could theoretically happen would be that if the A51m tried to draw too much power, you would trip a fuse or circuit breaker in your home for the entire circuit that was feeding that outlet.  But if that happened, then you would have to plug the two AC adapters for the A51M into wall outlets that were on two completely different circuits, and two power outlets right next to each other on the wall will almost always be on the same circuit, so using a splitter won't make a difference compared to using two outlets on the same circuit.  But again, that's theoretical.  I can't imagine an A51m tripping a household circuit breaker unless maybe you were also trying to run a microwave and a vacuum cleaner on the same circuit at the same time.

For reference, I run my entire home theater setup (75" TV, high-end AV receiver, 9-channel Dolby Atmos speaker set, 500W subwoofer, and various video source devices) all from the same surge protector plugged into a single wall outlet.  I've never had a problem even while listening at high volume.