@kurtmeadows in general I would avoid using this method, unless you really need a feature from that pathway (like dual link 444 SDI). Also yes this is limited to a single output. The Blackmagic cards have no processing capability, it will waste a lot of resources to go back and forth from the GPU (where your image is calculated), back to the CPU and out the Blackmagic output.
It seems your need is only monitoring what you output to the projectors, this can be done inside Isadora, or by splitting the output signals, or using a multiview monitor. I would go for adding an extra monitor to you gfx card and asking yourself a split screen preview of everything you need to see. Just use normal HDMI to SDI for your output (even Blackmagic sell these do 50 bucks these days). This way is less cabling and less work for your computer as well as only requiring a single screen for preview monitoring.