Double dose of Hep B vaccines

Our clinic routinely administers double doses of Hep B to our HIV+ patients as part of a 3 dose schedule. I believe that we should use 90740 instead of 90746. But how do I get paid for two doses instead of one? We have billed it out on two separate lines and tried billing it on one line with two units but insurance is only covering one dose.

I don't believe that modifier 59 is appropriate for immunizations and our billing software states that modifier GD is inappropriate. Any other ideas? Thank you for your help.

Double dose of Hep B vaccines