Actually the neptune space can support a second first stage input so you just plumb two bottles in and turn the correct one on. I can't say how well it works as I've only read about it. I wouldn't use the mask on a technical dive anyway. Way too burpy in my experience.they cause far more problems than they solve and I firmly believe they are a tool to be used only when necessary, i.e. contaminated water and when comms are required.
For things like deep diving they are properly annoying because you either have to deal with switch blocks *which are expensive, annoying, and highly risky*, or you have to come off of the FFM to switch to another bottle.