Advice on Buck-Boost Stage
Using converters in "cascade" is a waste of power. A simple solution is to use a single separate converter for each motor. In this way you resolve the problem noted by @AdamHaun in the comments. As your energy supply range can be lower or greater than the desired output voltages, you need a converter with step up and step down capabilities.

Some non isolated topologies can achieve this: "Non Inverting Buck-Boost", "SEPIC" or "Inverse of SEPIC". If you are ok with negative voltages you can use also "Buck-Boost" or "Ćuk".

Another advantage of using separate converters is that you can just design one single converter (and build it twice), since the specifications are not very different between the two in your application, this saves you money and time. Even another advantage is that if one converter fails the other may operate ok.

Isolated converters like the ones suggested by @jms are desirable if you need electrical isolation. But if you don't need it, it's better to avoid those converters as their design is more complicated. Also there are power losses in the transformer.