While this is good from the perspective of minimizing fees, it doesn’t allow you to easily maintain a fixed percentage of stocks based on your investment strategy as there obviously is no automatic rebalancing between the stock and cash accounts.
Very limited and manual rebalancing would be possible by (monthly or quarterly) adjusting new contributions. However, that would likely not be sufficient after a significant stock market correction or crash. If both accounts are at VIAC, you could change the strategy of the cash-only account to have cash and a bit of stocks to restore the overall investment strategy. However, this would still be a cumbersome manual action.
If you don’t have an investment strategy with a fixed percentage of stocks, it’s no concern. Otherwise, I wouldn’t recommend this approach.
If you don’t invest 100% in stock because of your current risk assessment/profile, you should contribute to both parts from the start (but see my comment above). However, for many people it makes sense to start with 100% stock in 3a and gradually lower the percentage as you get older. For that strategy your idea could make sense. It may not be optimal after a stock market correction towards the end of your contributions, though (at such a point you normally buy ‘cheap’ stock as part of rebalancing).