I have a defendant who just picked up his 6th DWI. His prior DWI convictions are as follows:

DWI Open Alcohol Container 2010, 3 days jail (M/B)
DWI 2009, 2Y probation, revoked in 2010, 63 days (M/B) (concurrent with case below)
DWI 2009, 2Y probation, revoked in 2010, 63 days (M/B) (concurrent with case above)
DWI 3rd or more 2010, 2Y TDC (F3)
DWI 3rd or more 2015, McNeely issue, reduced to DWI 2nd, 80D jail (reduced from F3 to MA)

In reading 49.09(g), I understand I can't double dip between that and 12.42(a) if I'm using the same offenses to prove up the enhanced DWI. However, can I use 2+ of the misdemeanor DWIs (2010, 2009, 2009, or 2015) to make it an enhanced DWI (3rd degree) under 49.09(b)(2), and then use the DWI 3rd conviction from 2010 under 12.42(a) and enhance it from a 3rd degree to a 2nd degree?

You certainly can and should

Agreed. Use all of the misdemeanors as your jurisdictional priors and the 2010 TDC trip as an enhancement paragraph.

Phillips says: "The statute specifically allows a previous conviction for an offense relating to driving, flying, or boating while intoxicated to be used to enhance punishment for repeat and habitual offenders. See Penal Code 49.09(f)." 992 S.W.2d at 493.