Just changed from a hand control to a foot control. Same TM brand and both 24V. The one that is coming off had 2 12v leads going into the plug and has a switch on the tm to change it to 24v. The new one is 24v input only. I changed the wires to be in series at the plug to get 24v and it works fine.

Here's my question. Would it be worthwhile to series it at the battery and double up the cables going to the front. That would make the cables twice as big and half as long of a total run?
Just wondering if there is any benefit? Thank you

Yes if you do that you will have more power to the trolling motor and batteries will have power longer due to pulling less amps.

You will still have a single ground conductor the same size so it really won't buy you anything. Your wiring diagram should look like this..

What this diagram shows is essentially what I want to do, but being that I have 2 sets of wires going from the batteries to the trolling motor I was going to double them up and put an equivalent sized wire between the batteries to complete the curcuit . I think that's what ill do. Maybe ill use a car battery size wire to go between the batteries.

Ok then you must have 2 negative (ground)(black) wires and 2 positive (hot)(red) wires. depending on the guage of the wire doubling up will carry same current but the conductors will carry less heat. At some point with a big enough guage wire size it would be a wash. Certainly 4.0 will carry better load than 6.0.

DC wire is sized for a certain percentage voltage drop over a specified distance, usually 3% for anything involving electronics. If the wire was correctly sized initially, then the increase in voltage is going to be negligible and it does not reduce the current draw through the wires. The TM is going to draw a given amount of current at a given speed.
Check your existing wiring to see if it's SAE or AWG. If it's SAE, then by all means change it to AWG. When I rewired my G3 I found they had wired the boat with SAE wire. SAE is 12-18% smaller than AWG and will carry less current and handle less heat than AWG.

What size wire. If you have 6ga in place, you're not going to gain enough to make a difference -- unless the wire is in bad shape. If you have 8ga, then yes, it is worthwhile to double up.

John