If anyone plans to take Ather for a road trip (even a short one like Mumbai Lonavala), they will need to carry portable charger. When you aren’t riding on the trip, you’ll need ability to stow both helmet and portable charger.
The boot has some additional pockets for small items like phone or small purse etc., not large enough for portable charger.
Has anyone been able to fit both in the boot space?
Yes, if properly managed, than it will fit… I have used it on a long trip once. Full faced helmet with portable charger
I own the old 450 and I can fit a full face helmet. A tiny half helmet. The portable charger. 2 phones and still spare some space for a few bananas 
But in the 450X you should be able to story the portable charger inside the full face helmet. It should JUST Fit.
Or you could do what I do. I carry 2 full face helmets sometimes I need to carry an extra duffle bag sometimes. So I end up filling the bootspace with my bag or one helmet and I have a small bike lock sort of thing that I use on my rear pillion grab handle. Opens up a lot of space inside the boot if you do that.
Here you go
You can also secure your helmet to the exposed frame under the seat, not as safe as putting it in the boot though. You can open the seat, tie the helmet clasp around the exposed frame and close the seat.