You can only contribute $18k as employee and $35k as employer (under 50 years old, anyway). Those totals are across both traditional and Roth 401(k) accounts. If you have 401(k)s at multiple, unrelated employers, you can get employer contributions from both to a total over $35k. But if both are Individual 401(k)s, I don't think that applies.
Yes. If you use both, you can work it so you hit the maximum 53k per year, or 59k per year if you are 50+, and have a big chunk in the Roth side so you won't have to pay taxes on the money later (You pay now, of course).
In theory, having a chunk in the Roth is a better deal. You pay taxes on that money now, but down the road, you don't have to pay when you withdraw. Assuming it has grown multiple times, you pay much less in taxes by doing it now. And, you are not forced to take mandatory withdrawals when you are older.
I understand the penalty for early withdraw, should you need to do that at some point, is more forgiving with a Roth as well.
