I agree, but disagree as well.
I understand what Phil is saying, but you can have to successful channels even when you're small. I have a vlog channel and a gaming channel, and I consider both to be successful and I never thought they'd both get to where they have today. On my vlog channel I have 278 subscribers, and on my gaming I have 50 subscribers. Two completely different audiences.
That being said, If you don't have time/knowledge to grow your main channel, you really shouldn't be making a second channel. Running one channel takes work and dedication, and if you can balance that on two channels, awesome!
I know i'm still in the improving stage. No way would I consider even one of my channels to be perfect. I know I still need to work/improve on my vlogging channel, so should I even have another channel? Maybe not. I do both channels for fun. Sure, i'd love it if I got big (don't we all?), but for the time being it's just a hobby that I love to do. To me it's not about how many subscribers you have, it's about the fans. The reason that you make videos. I don't think it matters if I have 10 subscribers or 100,000 subscribers. If I have majority loyal active fans, I'm happy.
I typed a lot more than I was planning on doing hahaha.
Long story short: If you feel like you can manage both channels, go for it. If not, wait until you are ready. It doesn't matter how many subscribers you have.