JPF Home Page Forums Musician Gear and Techniques Microphone and Vocalist Forum condenser microphone

Simply put, you're asking for the impossible with what you have. There is no way you're going to keep vocal volume on a consistent level when you turn away from the mic.
That said, and I mentioned above, at many of my gigs I'm 6" to 12" away from the vocal mic so if I look off to the side, or up and down a bit, including the fretboard, it doesn't lose all that much.
The more mics open the more chance for feedback.

Now, I often set up my system in my living room with multiple mics and can easily get a nice mic to guitar mix with no feedback, and at, if needed, fairly loud levels. That leads me to believe it's more likely your signal chain and settings.

Now, this ain't exactly right but will get you going better, I think....

Get the amp off the floor. Make sure it is NOT behind you.
Plug the mixer out/s into the amp's input/s. Set the amp volume to, say, 30% as a start. Leave the amp's eqs flat. Plug both mics into the mixer and set channel eq, gain and levels there. You may have to pan one channel hard right and the other hard left, they'll combine in the mixer and go to the amp as one. Start with the top knob, gain, at 50% on both channels. Turn the bottom, level, knob up as needed. Re-adjust the gain to bring up one mic or the other as needed.
Essentially you want the gain set to bring the mic signal just to the point where you start hearing it bring in noise, then back it off just a bit. Then, with your amp volume set where you want it, adjust the individual channel output levels to get out of that channel what you want and into the amp.
With a dynamic mic your gain may be up at 75% or more and a condenser may be below 50% and have the same relative volume going out. That does't always hold true however. I have a beyerdynamics condenser mic that is low in output, similar to many dynamics.

It still looks to me like a signal chain/gain structure problem, not an equipment problem.