This would work, if you could pick which money to convert - but you can't.
The IRS rules are that all money in all your IRAs are considered one big pot, and it is always merged together for tax consideration. That means, if you do any conversion, you convert always a proportional mixtures of your pre- and post-tax contributions, no matter how old they are, or in which specific IRA account they are sitting.
In other words, it wouldn't help you to have two separate accounts, one only with pre-tax and the other one only with post-tax money - it is still lumped together for taxing.
It could still be a good idea to do taxable conversions while the value is low, so you might consider converting a certain chunk or all of the mixture now, and pay the taxes while the amount is low. Of course, paying a lower percentage of taxes once you are retired might beat paying higher taxes on a lower value (while the market is down) - there is no definite way to predict which one is better.