Welcome to the forum.
Assumption is the inquiry is for retiring the active seal and putting the existing backup seal into the seal carrier.
The big issue on the 340DA is access.
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Work on my shafts and seal carriers was done several times by me on my 380DA. The 340DA has excellent access, no disassembly required.
It looks like the best way to gain access on the 340DA is to remove both inboard (above keel) exhausts, the hoses, elbows, risers and manifolds. Removal of those parts would permit a human body to get underneath the engines to perform that work.
My very strong suggestion is to yank the boat and do this work on the hard. It is theoretically possible to do this while floating, but my strong suspicion is that there will be massive water intrusion once the old seal is yanked.
Note that prior to yanking the boat, two tests are required. One verifies the pressurized water flow supplied by the engines to the seal, the other verifies that unfettered raw water ingress exists backwards through the seal. Verify per the seal manufacturer's specs. These are good to perform annually.
If either very important test fails then locate / correct the problem.
I always mark the location of the seal carrier on the hull tube so that I can place it at a different spot each time I service the seal. This puts the new seal to ride at a different spot on the shaft.
Assuming those seal tests are good, my preference would be to yank the shafts (do both even though only one is problematic) using a seal protector so the keyway slot doesn't kill the seal. Swap the existing spare seals to retire the existing active seals and install new spare seals.
Every time I yank my shafts I polish them prior to replacement. I put these across step ladder rungs and take long narrow strips of emery paper for long random motions to remove any crud and blemishes.
My boat has long since had both props, shafts and couplers cross lapped so that disassembly is "easy". When I got the boat both the shafts showed diagonal contact marks on the props and on the couplers and disassembly was a royal butt pain. I have since used coarse to fine lapping compound on these tapered connections to now have 100% contact patch and normal disassembly.
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Note that engine alignment is very important for decreased vibration and minimized seal wear. My alignment was tweaked by me from normal misalignment to dead-on balls accurate.
The cutlass bearings are also an important part of this system and should be checked for possible replacement.