Here is another example of a professional system for use in apartment blocks, in this case from Philips.
Philips Central Antenna System Electronics 193911.pdf
It is evident that at least in the distant past, quite a bit of engineering effort was put into the issue of feeding multiple AM receivers from a single aerial. More complex professional systems, such as those for apartment blocks, used distribution amplifiers, whilst simple systems for a small number of domestic receivers used passive methods. It is evident that in the latter case, the “splitting” was done at low impedance, say around 75 ohms nominal, with step-up transformers used to match the high impedance of typical domestic receiver aerial inputs, these transformers also providing the required inter-receiver isolation. The transformers would also ensure that the receiver aerial inputs were looking back into impedances that were somewhere near to what was expected and so did not suffer from undue detuning and/or desensitization effects.
Perhaps splitting could be done at high impedance from a longwire aerial by including a resistor, say 1k, in each receiver feed in order to provide some inter-receiver isolation. There would be some signal loss, but perhaps not unacceptably so if the aerial is “big” and provides a relatively high signal strength, at least on local transmitters, to start with. It might be worth a try.
Cheers,