You can just try it out and see how it goes! It works for some but not for others! It works for channel kiddyzuzaa and Disney Car Toys feature both boys and girls toys! However most top toy channels are niche specific! Videos that gets views and increase watch time quicker has a higher chance of being suggested and promoted by YouTube. When a video gets release and those first initials people that watch it (usually your subscribers minus those other channels trying to promote themselves) click away, Youtube will unlikely promote your video to other people. Since usually those first views comes from your subscribers, if most people subscribe to watch toy cars and you post Barbie, the yea most likely they will click off sending a signal to YouTube that maybe this video isn't so great. Of course it could be the greatest Barbie video ever but YouTube didn't know that! There are time where YouTube will test your videos suggesting to other doll videos and if it does well there, then it will promote it again! I know you're looking for a yes or no answer but it's really up to you! For me, I wouldn't combine it but running two channels is very hard starting out!