If you have multiple Kewl Tunes on the field, you should be able to use multiple Tuners from your hand for a Synchro Summon. As you can see below, Master Duel allows this but Omega does not.
Thank you for making a bug report.
Please keep in mind neither Master Duel, nor Omega are to be used as a ruling resources. In both cases there have been fallacies with the way a ruling has been programmed.
https://db.ygoresources.com/qa#24208
That being said, you are correct. Though I’m not sure if Omega would be capable of having this be fixed with a simple code fix. This might require a core fix.