I see this as your opportunity to have both girls become your MOH and share in the responsibilities that come along with this honored title. It isn't just a position they take in your wedding, there are duties that must be done to assist their bride. It is a priviledge, not a right of "who knows her better" to become the bride's MOH. With the 2 (or I have done 3 at one wedding) this will enable them all to assist you with so many of the tasks you will undertake while planning your special day.
Sit down and discuss these duties with them all and see if they are up to it prior to their acceptance. This will make everyone involved feel comfortable and that they are ideed needed.
Print out this list of duties and responsibilities for them. Take them out to lunch or cook for them at your home while sharing together. Let them know how much this means to you:

Bridesmaid 101 - Maid of Honor Duties

Everything will work to your advantage and your friends will thank you for being so thoughtful!
Jen