Virginia, I think they will only charge you one standing charge if you have 2 MPANs and you are on a single rate tariff ie. doesn’t matter what time of day you use your electricity you are charged the same rate. I am in same situation with Bulb and they refunded me money and gesture of goodwill recently, which was good as I didn’t even know anything about it. The only reason I have E10 is that the flat was built with it and Storage heaters and boiler are on the E10 meter.
Bulb currently charge me E7 on Day/Night meter and then a standard rate on E10 which is fine outside of winter, but expensive (ish) in winter. EDF have a complex meter team (0333 009 6980) with specific E10 tariff and one Standing charge. SSE used to not sure if still do. Others will quote you on their E7 tariff but in main you only get 7 hours cheap electricity which may not be enough to heat your home. With 2 MPANs unless you get a single rate tariff you will still be charged 2 MPANs from the smaller suppliers.