The 6668 has been around for a while and it is marketed for use with Advance Mark 10 Two Wire dimmer.  It also lists Philips Marathon and Earthlight, which are CFLs.

If the 6668 can dim those CFLs and Advance Mark 10 and 6673 can dim "dimmable CFLs", can I dim Advance Mark 10, Lutron TuWire, or OSRAM SYLVANIA PowerSense phase control two wire dimming ballasts with 6673?

I believe the 6668 has been around longer when Earthlight was the only dimmable CFL.  Since 6673 doesn't exclude Philips Marathon or Earthlight, it seems logical to be able to use 6673 with Lutron Tu-Wire and the like.  If I'm willing to adjust it using manual mode as given in instruction would it work?

In OSRAM SYLVANIA's dimming ballast sell sheet, it says

"2. Power-line controls must be rated for the type (e.g. Fluorescent Phase-control) and size
(e.g. 600W, 1000W, 1500W & 2000W etc.) of the connected load. Do NOT use incandescent
power-line controls; incandescent dimmers are not rated for fluorescent loads and are NOT
compatible with POWERSENSE ballasts."