Is there such thing as a sealed dual extruder hot end?
It doesn't work that way, or perhaps in some sense it already does.
The &quot;straw effect&quot; you're describing depends on the water not being under any pressure that exceeds the outside atmosphereic pressure. If you squeeze the straw or otherwise apply pressure, it will immediately spill out.
If you're printing at more than a ridiculously slow print speed (slow enough that it would fail for other reasons, like the nozzle melting the already-printed part just by proximity to it), the filament is under very high pressure from the extruder gear. The primary function of retraction is not to move the filament back out of the nozzle orifice, but simply to relieve that pressure. (A little bit more is needed to prevent oozing, however.) When retraction happens, it's exactly like the &quot;straw effect&quot; you're describing. The material pulled back out of the hotend into the heatbreak hardens enough that it makes a nearly air-tight seal, preventing the molten material below from flowing out of the nozzle due to gravity.
If your hotend does not have design problems and if you're using retraction correctly (that means using it everywhere that you're making a move that's not an extrusion one, not skipping it with &quot;Limit Support Retractions&quot; or &quot;Combing&quot;), you should not see oozing, ever. The system already works the way you want it to. But you can't magically get rid of the need for retraction. It's part of how the system you want works.