I aim for $450 for two; I wouldn't consider that particularly frugal, it's an intermediate goal. I think we can do $400 quite comfortably and $300 if we tried hard. $200, I think, would be enough of a stretch we would feel deprived (i.e. would do in extreme circumstances, not voluntarily). We are basically omnivorous and have some foodie tendencies; rice and beans just won't cut it here while we have an income. :)
On sharing accounts:
My personal experience with a joint household checking account has been very good. I think dealing with a shared pool of money has helped us get on the same page on managing household expenses, and I think waiting until our expenses added up to split them allowed us to be fair without being/feeling petty (Asking for $15.03 for the grocery bill, only to give $21.32 back the next trip seems very nitpicky to us, personally. I realize it works for others and that's great.) He's a user on one of my cards, which we use when we get groceries, gas, cleaning stuff, campgrounds/hotel rooms, tickets to events (to attend together), etc. Toward the end of the month I add rent+utilities+car insurance+internet+card and divide by 2 and we each put that much in the checking account, then everything gets paid from that. We're a month ahead on rent and utilities to prevent accidental overdrafts. If our spending seems high I look at the categories and we adjust the next month. I like this monthly system far better than per-transaction accounting.
That said, disclaimers/opinions:
-I wouldn't (didn't) share an account with someone I wasn't at least living with. (Married living separately would be ok, too). I don't think there's a real need for it; you don't have many joint expenses.
-I wouldn't cohabitate with an SO if we didn't *both* consider it a step toward marriage (not just the next step in the relationship, a way to save on rent, etc).
-It takes trust that your partner will handle a potential (if unlikely) parting of ways in a mature and measured manner, rather than clearing out the account and apartment and disappearing. On the flip side, of the same coin, it helps to go in with the mindset that you are taking on a degree of risk, because you are no longer 100% in control of what's happening with that account, and even the person we think we know and trust 100% has some degree of unpredictability to them...
-BY NO MEANS would I EVER pool ALL finances before marriage. That seems to be asking for trouble (in the 1% case your partner isn't who you believe they are). Also cosigning on a loan, joint credit card, anything else with large-sum, long-term consequences. My HS Consumer Ec teacher urged us to always have an independent account, even if we did do joint finances with a spouse. Personally, I've always thought that sounded prudent.
-Ok one exception to the above on joint loans: if you add an authorized user to a credit card (at least, with AmEx), you retain full responsibility for any balance they put on, but you also can set a limit on the balance they can run up and cancel their authorization anytime. As opposed to a joint card, where you both have the right to run it up to the credit limit and either or both of you can be held responsible for the full balance. Nice side effect, maybe: I've personally seen this help the authorized user's credit score, since your credit limit counts toward their available credit.
-I think pre-engagement couples should aim for 50-50 split on their finances at the lower earner's standard of living. Should the higher earner have a particular need/want beyond that standard they can cover it additionally, e.g. an apartment/home with an extra bedroom for an office if they WFH a lot. I think an income-proportional split in contributions to various things can make sense for married couples, but pre-marriage it amounts to a subsidy that might just disappear. Engagement can sort of go either way, depending on how seriously it's taken. User's choice.