TBH having a release video is optional in the first place, but it certainly helps with visibility (which is I assume your goal here).
If the bank has multiple features you want to show off it can be good to make a demo reel, like for example, since its multipitch, you might make one cover (or partial cover) to show off its high ranger versus one for its low range, but otherwise usually just having one release song that adequately shows off the VB's capabilities is enough to get potential users interested.
Releasing multiple voicebanks within a short timespan can also help garner interest for your UTAU, but I think still giving them separate release videos is better; for one thing, if you want to play the algorithm game, putting out multiple similar videos is better for channel growth and visibility, plus it helps with searchability for people looking for new voicebanks in the specific language, and it makes it easier to link to a demo for a specific VB. You can always link to the other voicebank releases in the video's description.
For a more subjective reason, I find that if I tell myself "I can't put out X project until Y project is also done" it makes me less likely to finish and/or release either of them lol
