well if your partner is a SAHD, and you are seeing them for a reliable 2 hrs a night, this sounds incredibly sad to say, but they'll be seeing their parents a hell of a lot more than most of the friends and family in my circle
in most cases of family, there's 1 SAHP and a (major) breadwinner parent, mostly dads, who literally never see their kids mon-fri and then they're away on business trips some weeks too. so it's weekend parenting really, despite being in marriages. and then there are friends who have kids, apart from a couple most people i know have 2 workers, some mums part time, 1 dad part timer - they also have shockingly little time with their little ones when you think about it in detail like the OP.
really sad, but there's not much else to be done - bills still have to be paid, mortgages met, pensions and tuition fees saved up for - there is no alternative. just focus on using those 2 hours for quality time and do housework etc once they're in bed - that's the ONLY way I've found of coping (as a mum who has been the main breadwinner with a SAHD parent, and earlier on as the SAHM with DH working until 9 or 10pm most nights - basically a dad on weekends only).
for some reason if you point out the lack of family time in the traditional nuclear families (SAHMs, working dads) most friends and family don't see an issue with it, but when it's the mum working untl 9 or 10 at night, then it's seen as a problem - as if dads bonding with their kids and getting to miss all the important milestones is acceptable, but with mums it;s more painful - speaking on behalf of my DH, it isn't.
depressing topic, eh!