Yes, it could be done, but it's no easy task and would be impractical.
The alternators would require identical internal construction, they would have to be properly phased (and remain locked to that phase) therefore they couldn’t be belt driven as slippage and belt travel would alter their phase, so they would require chain or gear drive and must spin per identical ratios, AND they would require a 24 volt DC source such as a 24 volt DC battery to produce their exciter coil current so they could begin to output voltage. Additionally, they couldn’t have self-contained regulators nor the rectifying diodes, as they would need to be constructed externally for this 24 volt construction.
Considering all the above factors, it would be far less complicated and far less expensive to simply purchase an alternator that was designed to output 24 volts. Hope this helped.