Is it possible to combine multiple step up inverters in parallel?
No, it is not a good idea to connect DCDC converters in parallel unless they're designed for that.

If you used DCDC converters in parallel anyway then there is no way that you can make sure that all parallel converters share the load current equally.

The converter which is set to the higest output voltage will take all the load and all the other converters will not do much or anything at all.

If:


converter A is set to 20.0 V
converter B is set to 20.1 V
converter C is set to 20.2 V


And all their outputs are in parallel then converter C will "win", the combined output voltage will be 20.2 V. Converters A and B will not do anything as their feedback circuit will notice that the output voltage is too high (higher than what they are set to). Only when the voltage drops below 20.1 V or 20.0 V will they start to "work".

So when the output is loaded converter C will do its utmost to keep the voltage at 20.2 V. So if the load is drawing more current than converter C can deliver, converter C will be overloaded. What then happens depends on how the converters handle an overload.

The converter might "do its best", work at 100%, get hot, it might shut off if it has overtemperature protection. Only if the voltage drops below 20.1 V will converter B start helping. Even when it does, converter C still works at 100% while A does nothing.

Even if you say: "But then I'll just set them all to 20.0 V exactly" that will not work because there will always be small differences which will result in the same behavior.

Only if there's an additional circuit present which adjusts each converter such that they share the load equally, can this work properly.