My BatteryMINDER charger says that I can charge more than one battery in parallel.

http://www.vdcelectronics.com/batteryminder_12117.htm

I was thinking of wiring a connector between my 2 batteries (cranking and TM) so I could have them wired parallel during charging and maintenance modes. Of course I would disconnect them before use.

Would this wiring idea also work in a pinch should my cranking battery ever go dead and I still had TM battery charge to start?

I am pretty much electronically "challenged"...

Yes, but you should make it out of at least #2 guage wire to handle the amps it would be pulling during cranking

You can mount and connect the Battery Minder in the boat, leave it connected to one battery, add a battery disconnect switch between the two batteries and all connections would be premenant, all you would have to do it close the switch when in maintenance mode and open it during operation.  

For just a few dollars more you could add a two bank BPS or Cabela's two bank onboard charger and be done with all of it.  The ProSport II 8 is about all you would need and is only $100.  http://cabelas.com/cabelas/en/templates/product/standard-item.jsp?_DARGS=/cabelas/en/common/catalog/item-link.jsp_A&_DAV=MainCatcat21276-cat600056_TGP&id=0043939018868a&navCount=2&podId=0043939&parentId=cat600056&masterpathid=&navAction=push&catalogCode=IH&rid=&parentType=index&indexId=cat600056&hasJS=true

Didn't check BPS, they may even have a better price.  Both brands are made by ProMariner and are excellent charges.  Just because it say's it's just an 8 amp, don't let that fool you.  They don't work like other 8 amp chargers with just four amps to each bank.  With their on demand charging system, it charges the cranking battery at 8 amps and then switches to the TM battery.  It will charge almost as fast as the Dual Pro 20 amp charger.

I agree with Way2slow, get the onboard charger. It wis a better choice. just plug it in and forget about it. I have 2 of  the BPS XPS charger 3 bank 15 amp ( made by Pro Mariner) its a great charger charges the batteries fast and keeps the in top shape. If you worried about the starting batt going dead, keep a pair of jumper cabel on board this way if it does you can use you TM battery to start the motor. I have done this alot in the past, when I only had 2 batteries. If your thinking about getting a 24 volt TM in the future I would say go for the 3 bank charger now. Its only $30-$50 more than the 2 bank and you can hook 2 leads up to one battery now till you get another battery.

BP prices

XPS 2 bank 8 amp is $99.99

XPS 2 bank 10amp is $119.99

XPS 3 bank 15amp is $149.99

Look on line and in the mags, if there is a BPS or Cabela's by you check it out. You might find them onsale. I got one of my 15amp 3 bank charger on sale in the store for $119.99 2 years ago. and bought my Dad the 2 bank cahrger for $69