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My thoughts. I wouldn't be game to install a Paralleling Switch for House Batteries & Engine Start Battery. The only time I can think something like this may be required is when the engine start battery is flat and you need power to start the engine.
If the engine start battery was only just short of enough grunt to start the engine then its terminal voltage may still be reasonable and if the house batteries were connected to it, the inrush current would not be too scary. But if the engine start battery was flat due to an internal fault/short, then connecting a good set of say 800A house batteries to it has the potential to unload a lot of current in a short time into the fault. The result could be quite scary and a loud noise, molten lead and acid could appear from nowhere!!.
I have 5 x 160A batteries and each battery is capable of unloading 1,600A for 5 seconds. That's a combined total of 8,000A at you finger tips. If you have a paralleling switch on your boat, just make sure you know what you are connecting this beast to when you rotate that little paralleling switch (probably rated about 400A)
On my boat, there is no connecting between the house batteries and engine start battery positive terminals, but the negative terminals have been commoned up. I have a change over switch installed which allows the engine starter motor (and engine circuits) to EITHER receive power from the engine start battery OR the house batteries but never both at the same time.
Also my engine battery is sealed lead acid and the house batteries are AGM which have different charging requirements.
I saw in another forum topic where the guy requesting information on the electrics for a winch was advised to connect the positive terminals of his house and engine start batteries. I hope the information I have just provided my enable a more informed decision be made before doing this connection.
My thoughts anyway 
I fitted one of these blue sea
www.bluesea.com/products/7610/SI-ACR_Automatic_Charging_Relay_-_12_24V_DC_120A
www.bluesea.com/products/8690/Dual_Battery_Bank_Management_Panel
So far not a problem works well with my solar panel keeps both batteries topped up 120 amp each deep cycle cat batteries.As Im parked up for a while yet do not need any more house batteries. keeps every thing going in the boat.
Ordered two 'hundred meter rolls of Tyab tinned wire last week should be a lot of spaghetti running around the boat in the coming ,months
Ill be wiring in a fridge freezer unit next month it will be interesting to see how much power it uses.
wow, you have lot of Amps, serious power station, but
do you have any wiring which can carry those Amps, probably Not.
well, they are safe and stored in the batteries.
guess the only starter and winch motors are hungry for extra Amps.
Did you have ever need to charge house bat. from engine ?
Probably not, why bother to install it now, if you engine bat. dies,
disconnect eng. bat. , reconnect to the house until you get replacement.
Sure, I have switch between house and engine bat. but they are the same capacity
and the same way engine charging them.
Each system, boat wiring is different, people mostly referring to own boat.
In most cases works well, also can be improved.
God, it's all beyond me. I really must read Nigel Calder's Mechanical and Electrical Manual that is sitting on my coffee table. 
DrRog..reading might by more confusing, typicaly explain basic principles with simple examples.
just went through above mentioned ACR, one schematic shows a fuse for the house,
next schem. and the same page completly omitted.
Which one is correct, even profesional product is full of mistakes.
HG ..nice panel,, like your bat. switch with "combine batterie position " that's
what you try achive.
ACR sound like a good idea but proper function on the boat for extended
period of time would be doubtful.
Terminals A & B are interchangeble, means if one of batteries drops under
10 volts, ACR would work.
If control line /name Start Insolation / would not get sufficient voltage drop,
,they specify current 15mA, ACR would not work.
in FAQ..they mention ..it might happen even when batteries are not fully charge
because of voltage drop is not sufficient .!.
ACR is design to carry charging current only...why 2 fuses and wiring 150 times overrated.
......it only get some charging current for one battery only, never starting or house current ....
The way they describe the unit, you excused, they have mistakes.
If you not sure ask, avoid future troubles.
Guess, you have that under control. if you have any discrepancies ..ask.
Proper function of ACR....first set up monitoring using LED
second using solar cut of switch and the right connection of control line of ACR should work ok
third..before you finalise wiring and ACR playing up, before you leave the boat, connect
both pluses with jumper, solar cut off takes care of it, both bat. will be fully charge all the time.
when you get on boart, remove the jumper and monitor proper function of ACR
Before you star any new wiring would be good idea to draw all required circuitry.
