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There are may more knowledgeable electrical folks on this site, but I believe there are disadvantages in the approach you are considering. I have dual tender from PulseTech:
The features of this tender and likely other dual tenders, is it can “individually” and continuously test and apply recharge to each battery only as needed. This continuous test and judicious application of recharge extends the battery life. Again I am not a skilled electrical guy, but I believe the approach you are considering is suboptimal to the primary battery which you will have connected in a series.
I have a dual battery tender for use in the off season. It is the model from battery tender. It’s not cheap but it works flawlessly and keeps the trickle on both batteries.
If you've got power, go ahead and get a two-channel ProMariner prosport (or something similar). I don't even think about it, plug in when done and two nicely charged batteries the next time I go out.