Did you buy the ticket within the last 24 hours and is the trip happening in over a week? If so, you should be able to cancel free or charge per DoT rules. Yes you can split PNRs at any time either online or via the call centre. Does your husband have status with UA? If so, then splitting the PNRs will result in you losing any benefits you get from flying with him (i.e. free E+ seating, free day of departure changes, etc.)

Otherwise, if we're past the DoT rules, then, and this is gonna sound strange, your best bet may actually be to fly the SFO > EWR segment initially and then make the change after you've taken the outbound flight. IIRC UA prices changes to flights differently depending on whether you've flown any of the trip or not with more favourable terms coming if you've flown parts of the trip already. I think if you change the return segment after flying the outbound segment it should work out to just a change fee with a very minor fare difference, basically is there is any availability of the new flight in the fare bucket you purchased. Switching the origin on the return may complicate matters so I'll leave it to the peanut gallery to comment on this one.

In theory you can cancel out the trip now or sometime during the travel and you'll get a credit equivalent to what the fare cost for future travel. However, when you use that travel credit UA will deduct the change fee from the ticket. I wouldn't recommend no showing since you won't gain anything out of doing so (may as well try to get some of the travel credit back).

Hope that helps.

Cheers,

James