What you are proposing is not a very good idea. Many of the reasons why have already been listed above. Another is different temperatures of the batteries will result in different discharge and charging rates, too. Either way, you are going to abuse these batteries and the result will impact their longevity and available AMP Hours and increased charging time.
Also, I would not invest in expensive batteries for this use. What is going to happen is shorter longevity. So, pay more or pay less with the results being the same, shortened life.
Personally, I would purchase two batteries that fit the existing location and go with that. Spend the money to purchase a completely separate battery for trolling use. You are going to end up spending the money that otherwise would have purchased this battery in the long run.
Good luck and thanks for bringing this question here so others can learn from your questions.
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08 F550-4X4-CC-6.4L Dsl-206"WB GVWR17,950#
09 Lance 1191
1,560wSolar~10-6vGC2-1,160AmpH~Tri-Star-Two(2)60/MPPT~Xantrex 2000W
300wSolar~2-6vAGM-300AmpH~Tri-Star45/MPPT~Xantrex 1500W
16 BMW R1200GSW Adventure
16 KTM 500 EXC
06 Honda CRF450X
09 Haulmark Trlr