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Either transmitter or spg!
I have a DS4 on my left post with a SPG and a DST with a Suunto transmitter pointing down on my right. This means the transmitter isn't pointing out waiting to be broken.
To do this with two DS4s would require them both being orientated as per the left post and I think that would block the isolator somewhat.
The spg goes to the right post and the transmitter to the left post. You might need to buy a longer HP hose.