Tell them you would love to reduce the amount of people the guild needs to run a raid, but you need their help, and understanding, while you learn the ins and outs of playing two characters at the same time. Grab some guildmates and head into any instance where a single simple mistake won't instantly wipe the whole group. Try to get a commitment of at least one instance run a night, and that everyone understands the purpose of these trips is for you to learn. Make sure they understand you will be making mistakes, but set some clear goals that you and your guildmates know about.

For example, in three weeks time you will be able to consistently main heal without error and perform in the top 2 dps for a 5 man group. In 5 weeks time you will provide healing assistance without error and be in the same dps spot you were with your hunter before for raid instances. Extra points if you get guild leadership to take ownership of this project. See if you can get a couple more people to adopt the dual-boxing approach and train together.

If it doesn't work, you can tell your guild you tried it, but you weren't suited for it. You may find that by just pushing the issue back to the people that are calling for it makes them stop asking. Everyone loves to complain, but it is not uncommon that when you ask for some time to sit down with them and go over their issues they never call you back.

Make sure they understand that your ability to play two characters at once doesn't reflect the viability of the concept.