There's a lot of synergy in doing both.

Battery EV is high-capacity battery + engine + thermal management system for the battery.

Hydrogen EV is fuel cell + low-capacity battery (for smoothing out peaks and recuperation) + engine + thermal management system for the fuel cells.

BEV have low price per km (here it's about 1/4 compared to gas cars), but lower range, while H2EV have higher price per km (in Germany where there's actually infrastructure available, its about the same price per km as gas car) but higher range, so it can make sense for some use-cases, like long-range buses or whatever.

You could even build Plug-in Hydrogen EV with medium capacity battery and have BEV-like usage where you charge at home but still get 1000 km range if necessary - it seems needlessly complex, but it just means adding bigger battery and charging port to H2EV, so it can make sense.

Another potential advantage of fuel cell cars is a lower purchase price. Toyota thinks FCEVs might become cheaper to buy than even ICE cars:

https://www.thedrive.com/tech/26050/exclusive-toyota-hydroge...

It might be in the future. Here (EU) and now (2019), they cost as much as BEV, while fuel costs as much as fuel for ICE, giving you the worst of both worlds.