Will playing Mass Effect multiplayer from one account affect every profile on the console?

My husband and I are using one Xbox 360, but different profiles. I want to play multiplayer in Mass Effect 3, but the Xbox Live account belongs to my husband.

The only thing I want from the multiplayer is to increase my Effective Military Strength in the single player game, but I'm not sure if it will affect the game on my profile if I use his account.

You have to use the same profile your saved games are under.

If you normally play single-player on your own profile, you have to use it for multiplayer as well.

You can get the Xbox Live Family Pack to save some money on two subscriptions, or try to catch a sale. (Or just pay for a month, get your multiplayer time in, and then cancel it.)