They could sell the dock and handheld separately to keep costs down, but also to be able to give the dock its own SoC so it can function as a stand-alone console, which can also be combined with the handheld.
It could work like this, with three configurations:
1. stand-alone handheld; 2TF and 1080p screen; renders games internally at 540p or 480p and upscales them to 1080p using DLSS
2. stand-alone dock; same chipset as handheld but with higher clocks, 4TF; runs games internally at 720p and upscales them to 1440p or even 4k using DLSS
3. dock+handheld combined; handheld runs at higher clocks in dock mode, raising its performance from 2TF to 4TF; thus combined performance of 4TF(dock)+4TF(handheld) for a respectable 8TF; runs games internally at 1080p and then upscales them to full 4k
I pulled the numbers out of my ass of course, but I think 2TF for the handheld is probably the most we can expect. It’s possible it‘s something like 1.5TF instead, which means the dock would be 3TF and combined it would be 6TF. As for my resolution numbers, while using such low resolutions as the base for DLSS doesn’t look ideal on a monitor or TV, on a handheld screen this would be much less noticeable, so they can go all out with using low internal resolutions for DLSS. By rendering at low resolutions like 540p (or even lower like 480p), the handheld would be able to punch far above its weight, and might thus receive lots of games that are on PS5/XBS (imagine what the PS4 would be capable of if it would only have to render at 480p and then use DLSS to upscale to 1080p).
And they could also make both the dock and the handheld backwards compatible with the previous dock and handheld. So you can put the old Switch handheld into the new dock to play Switch 1 and Switch 2 games on tv. And while Switch 2 games can‘t be played on the Switch 1 handheld natively (unless they‘re cross-gen), there might be a remote play option where the new dock connects to the old handheld wirelessly. Alternatively, you put the new Switch handheld into the old dock to play Switch 2 games on tv. So it wouldn’t be necessary for everyone to buy both the new dock and handheld if they still want to use the hybrid functionality (since many people already have the old dock and handheld). Though in this case they‘ll obviously only have access to the power of either the new dock or handheld (so 4TF max). And future hardware iterations would of course also be 100% backwards compatible; so if ‚pro’ versions of the Switch 2 handheld and dock come out at some point, you can just buy one of them to combine with the regular Switch 2 dock or handheld you already have.
As for price, I think the dock would be the cheaper of the two, since it‘s the same internal hardware but without a screen. 299 sounds about right, just like the Series S. The handheld will be pricier, and considering the OLED is 349 already, I think 399 isn’t out of the question. There might be a combined SKU too. The big question mark is whether they‘ll have some kind of SSD or not. If the Switch 2 still only has regular slow storage, while the other systems have their blazing fast SSDs, it‘s gonna feel rough.