Hi,
Generally some suppliers can support some Two MPAN tariffs but the heatwise is a killer due the amount of registers on it. I'm sure EDF &/or Npower can take on these as am sure I've seen them supply before but give them a phone and advise you have two MPANs and the Tariff is called Heatwise and can they support and they will soon tell if they can or not and give you any rates. Dont go near Scottish & Southern, Scottish Power or British Gas as they are dead certs they cant do it. Also a good idea is maybe try companies like Telecom plus or Utilita as they are offering not bad rates for electric only but maybe able to support your heatwise. I know some companies offer discounted rates for online maybe Eon offer this and it sometimes helps save money.
Kind Regards
Mr MPAN