There must be a lot in the archives on this.
The short answer to the one box or two question is that it depends, just like it depends for amplification components. There are advantages to having everything in one box--no duplication of power supply parts, fewer connectors, and so on. Having two boxes makes upgrading a bit more straightforward and isolates circuits better.
Most CDPs have a digital out so you can use them as transports with an external DAC. That's one way to upgrade but you must have a good interconnect to get the best out of it.
I don't know your player nor what DACs or transports you are considering. I have upgraded DACs twice. Over five years, the steps were 1) transport + DAC, 2) digital cable upgrade, 3) DAC upgrade, 4) sell all for one-box player, 5) add external DAC, 6) sell one-box player for transport upgrade, 7) cable upgrade. Each time, I got what I was looking for: a more precise and detailed soundstage, and a less aggressive top end.
The short answer to the one box or two question is that it depends, just like it depends for amplification components. There are advantages to having everything in one box--no duplication of power supply parts, fewer connectors, and so on. Having two boxes makes upgrading a bit more straightforward and isolates circuits better.
Most CDPs have a digital out so you can use them as transports with an external DAC. That's one way to upgrade but you must have a good interconnect to get the best out of it.
I don't know your player nor what DACs or transports you are considering. I have upgraded DACs twice. Over five years, the steps were 1) transport + DAC, 2) digital cable upgrade, 3) DAC upgrade, 4) sell all for one-box player, 5) add external DAC, 6) sell one-box player for transport upgrade, 7) cable upgrade. Each time, I got what I was looking for: a more precise and detailed soundstage, and a less aggressive top end.