Mostly, you will receive some kind of benefit or incentive to have dual fuel with the same supplier.
Some suppliers will have a national price for gas but all have regional for electricity (transco do the majority of gas to the whole country, however the distribution companies for electricity, plus the companies used for meter operations vary from area to area and therefore so do the costs)
This means the cheapest electricity provider for your area might not be the cheapest in another (hence why you need to put your postcode in these comparison sites so they can get the right prices for your area)
It may be the cheapest gas supplier has higher electricity prices for your area and any discount is counteracted by the unit prices, giving the results you got.
If you think you've found a better deal, go for it would be my advice. That said, if the £20 a year you'll save means going to a poorly rated company, also bearing in mind you'll need to deal with 2 separate companies instead of just one, is it worth it? If you've reduced your usage but were quoted on the old usage, the savings between dual an two singles will also be less.
Fixed price is always a risk, you pay more so you may not have to in future.. I don't think prices will come down personally, however I've not fixed mine yet either.. I think we'll know more by Dec/Jan